Opinion by
Mr. Justice Bell,These cases involve the same basic question and were argued together and will be treated as one in this opinion.
The question involved may be narrowed to whether §2 of the Corporate Net Income Tax Act of May 14, *1621947, P. L. 232, in its application to taxpayers’ 1944 corporate net income is constitutional.
Each of the taxpayers is a Pennsylvania corporation; each is engaged in both intra-state and interstate business; and each was subject to Pennsylvania’s Corporate Net Income Tax for the years 1944, 1945 and 1946. Each taxpayer duly filed with the Department of Revenue its corporate net income tax report for the year 1944 and each paid the tax shown therein to be due. In 1946 each of the taxpayers suffered a net operating loss which was allowed by the Commissioner of Internal Revenue and was applicable under the (Federal) Revenue Act of 19)2 to the taxpayer’s 19)) income.
The Corporate Net Income Tax Act was first enacted by the Act of May 16, 1935, P. L. 208 for a limited period of two years. It was thereafter reenacted up to and including 1945 (with amendments not here relevant), by every subsequent regular session of the legislature. It was denominated a state excise tax. Section 2 of the Act of May 7, 1943, P. L. 217 (and of the Act of April 11, 1945, P. L. 190) provided: “Section 2. Definitions. — The following words, terms, and phrases, when used in this act, shall have the meaning ascribed to them in this section, except where the context clearly indicates a different meaning. . . .
“ Net Income.’ 1. . . . net income for the calendar year or fiscal year as returned to and ascertained by the Federal Government * . . .”
Section 3 of the said Acts provided as follows: “Every corporation shall be subject to, and shall pay for the privilege of doing business in this Commonwealth, or having capital or property employed or used in this Commonwealth, ... a State excise tax ... at the *163rate of four per centum per annum upon each dollar of net income of such corporation received by, and accruing to, such corporation during the calendar year . . . one thousand nine hundred forty-four.”
The Commonwealth ignores the definition of “net income” and limits it to the net income actually received by the corporation during the calendar year. We do not agree with this construction.
The Commonwealth adopted as the base of its tax for determining the tax liability of corporations for the years 1943, 1944 (and by the Act of 1945 supra, the years 1945 and 1946), the amount of net income “as returned to and ascertained by the Federal Government.” Because of the losses which would likely be incurred in converting from wartime to peacetime business, the Revenue Act of 1942 provided that net operating losses could be carried back for the two years immediately preceding the loss year. That meant that each of these taxpayers could, under the Revenue Act of 1942, carry back its net operating loss in 1946 and apply it in reduction of its 1944 income. For the years 1941 to 1945 inclusive, the Department of Revenue and the Department of the Auditor General, in making corporate net income tax settlements and re-settlements, allowed deductions for net operating loss carry-backs to the extent that corporations were permitted to do so under the Federal law.
The Commonwealth by Act of May 14, 1947, supra, amended the 1943 (and 1945) Acts by providing, inter alia: “Section 2. . . . And provided further, That on reports filed for the calendar, year one thousand nine hundred forty-sise, . . . or any calendar or fiscal year thereafter, no deduction shall be allowed for net operating losses sustained by the. corporation during any other fiscal or calendar year, nor shall any net operating loss sustained by:the. corporation during the cal*164endar year one thousand nine hundred forty-six, ... or in any calendar or fiscal year thereafter be allowed- as a deduction for any prior calendar or fiscal year . . . .”
“Section 13. This Act shall become effective immediately upon its final enactment. . . .”
The Commonwealth contends that this amendment as applied to 1944 corporate net income is valid and constitutional, and that it made no change in the tax base of the prior Acts so far as carry-back losses are concerned.
The first question that arises is, what is the meaning of “net income” as used in the Acts prior to 1947 and more particularly do they permit carry-back losses sustained in 1946 to be deducted in ascertaining 1944 net income?
Pennsylvania chose and adopted as the base for determining the tax liability of corporations for the years 1943 to 1946 inclusive, the amount of net income as returned to and ascertained by the Federal Government, with full knowledge that The Revenue Act of 1942 allowed carry-back losses for the two years immediately preceding the loss year. But Pennsylvania went much further than this. During the month of June, 1946, in order to attract business to the Commonwealth and to retain within the Commonwealth as much corporate business as possible, Governor Martin and other high officials of the State advertised in the leading newspapers on the Atlantic seaboard as follows : “There are provisions for carry back and carry forward of corporate losses for purposes of State corporate income tax calculations which result in an effective average method of determining taxable income.”
Governor Duff, in his Budget Message in January, 1941, recommended a change in the corporate net income tax in order, to obtain increased revenue. He *165said, inter alia: “. . . One of the principal reasons [for the lower than anticipated receipts from the corporate net income tax] is that this State allows the carry-back, carry-forward of losses as provided in the Federal Corporate Income Tax. Under this provision losses incurred in the current year may he carried hack two years to be established as credits against taxes paid in these years or may be carried forward two years similarly. ...
“The carry-back, carry-forward provision was made a part of the Federal Tax Act in order to help to spread the burden of the high Federal taxes. Pennsylvania in using the Federal base did not exclude the operation of this provision as has been done in some other states. Of all the states having a corporate net income tax, this State alone permits the operation of this provision.”
Moreover, each of the taxpayers, in reliance on the law, i.e., the Act of 1943 and the Act of 1945, and on the Commonwealth’s advertisements and its practice prior to May 14, 1947, of allowing deductions for operating loss carry-backs to the extent permitted under the Federal law, made vital decisions and commitments, and in connection with mergers and a sale of debenture bonds and preferred stock, issued to its stockholders (in connection with possible purchases of stock), and to the buying public and to the (Federal) Securities and Exchange Commission, statements of taxpayer’s right to carry-back operating losses for the two years in question.
Taxpayers contend that if this Act of 1947 is interpreted retroactively, as the Commonwealth contends it should be, the refusal to allow the carry-back to 1944 of a net operating loss sustained by the corporation in the year 1946, is unconstitutional for the reason that it .violates (a)..-the. Due Process Clause-of *166the Fourteenth Amendment (Article XIV, §1) of the Constitution of the United States, and of Article I, §§9 and 10 of the Constitution of Pennsylvania, and (b) the Equal Protection Clause of the Fourteenth Amendment of the Constitution of the United States and the Uniformity Clause of Article IX, §1 of the Constitution of Pennsylvania, and (c) the Impairment of Contracts Clause of Article I, §10 of the Constitution of the United States, and of Article I, §17 of the Constitution of Pennsylvania.
It would be regrettable if the law required us to sustain the Commonwealth’s present contention that the Act of 1943 never permitted any carry-back losses in the computation of corporate net income and that the 1947 Act should be retroactively applied, because it would constitute an obvious breach of faith by the Commonwealth and be very unfair and unjust to the public and the stockholders who bought stocks or bonds of the taxpayers (on the faith of important statements which taxpayers properly made in reliance on representations of the Commonwealth’s highest officials) as well as to the Securities and Exchange Commission and to each of the taxpayers here involved. We recently decided in Commonwealth v. Western Maryland Rwy. Co., 377 Pa. 312, 320-321, 105 A. 2d 336, upon somewhat similar facts, that the Commonwealth could not be estopped by the public statements or official acts of some of the Commonwealth’s highest officials. We need not decide whether that principle would apply to the facts in the instant case, because the 1947 Act if retroactively applied to 1944 net income, clearly violates both the Federal Constitution and the State Constitution.
The Act of 1943, as reenacted in 1945, being a tax act and not an act providing, as the Commonwealth contends, for exemptions or deductions, must be strict*167ly construed against the Commonwealth and all reasonable doubts must be resolved in favor of the taxpayer: Allentown School District Mercantile Tax Case, 370 Pa. 161, 171, 87 A. 2d 480; Murray v. Philadelphia, 364 Pa. 157, 71 A. 2d 280; Commonwealth v. Repplier Coal Co., 348 Pa. 372, 35 A. 2d 319.
This Court has already decided that the prior Acts, i.e., prior to 1947, adopt as a tax base the “Net income as ascertained [by the Federal Government] .... How it was fixed by the Federal authorities is of no concern to the taxing officers of the Commonwealth nor to its statute.” Commonwealth v. Warner Bros. Theatres, Inc., 345 Pa. 270, 27 A. 2d 62; Commonwealth v. Electrolux Corp., 362 Pa. 333, 339, 67 A. 2d 105. We reaffirm the Court’s prior construction of the words “net income” as ascertained by the Federal Government; and we now hold that this necessarily means gross income less all the deductions and carry-back losses ascertained and allowed taxpayer by the Federal Government under The Revenue Act of 1942.
We agree with the taxpayers’ contention that the Act of 1947, by its terms and in its operation and effect, arbitrarily and unreasonably discriminates against them and therefore violates the Uniformity Clause of the Pennsylvania Constitution and the Fourteenth Amendment of the Federal Constitution which • — so far as reasonableness of classification is concerned, or as applied to questions of taxation — stand in pari materia: Allentown School District Mercantile Tax Case, 370 Pa., supra; Commonwealth v. Fireman’s Fund Insurance Co., 369 Pa. 560, 565, 87 A. 2d 255; Commonwealth v. Girard Life Insurance Co., 305 Pa. 558, 562, 158 A. 262.
In Allentown School District Mercantile Tax Case, 370 Pa., supra, we said (pages 167, 170) : “Article IX, section 1, of the Constitution of Pennsylvania, pro*168vides, ‘All taxes shall be uniform, upon the same class of subjects, within the territorial limits of the authority levying the tax, and shall be levied and collected under general laws; . . .’ This means that the classification by the legislative body must be reasonable and the tax must be applied with uniformity upon similar kinds of business or property and with substantial equality of the tax burden to all members of the same class: Commonwealth v. Girard Life Insurance Co., 305 Pa. 558, 158 A. 262; Knisely v. Cotterel, 196 Pa. 614, 46 A. 861; Dufour v. Maize, 358 Pa. 309, 56 A. 2d 675; Commonwealth v. McCarthy, 332 Pa. 465, 3 A. 2d 267; Dole v. Philadelphia, 337 Pa. 375, 11 A. 2d 163.
“. . . While taxation is not a matter of exact science and perfect uniformity and absolute equality in taxation can rarely ever be attained (Wilson v. Philadelphia, 330 Pa. 350, 352, 198 A. 893), the imposition of taxes which are to a substantial degree unequal in their operation or effect upon similar kinds of business or property, or upon persons in the same classification, is prohibited: Cf. Com. v. Overholt & Co., Inc., 331 Pa. 182, 190-191, 200 A. 849; Pollock v. Farmers’ Loan and Trust Co., 157 U. S. 429, 599. Moreover while reasonable and practical classifications are justifiable, where a formula or method of computing a tax will, in its operation or effect, produce arbitrary or unjust or unreasonably discriminatory results, the constitutional provision relating ■ to uniformity ■ is violated: Turco Paint & Varnish Co. v. Kalodner, 320 Pa. 421, 184 A. 37; Hans Rees’ Sons v. North Carolina, 283 U. S. 123.”
The 191fl Act amended, as we have seen, the’ definition of net income in §2 of the prior Acts, and in its relevant provisions provides: . . nor shall net operating loss sustained by the corporation; during the *169calendar year one thousand nine hundred forty-six . . . be allowed as a deduction for any prior calendar . . . year.” Section 7 of the 1948 Act and of the 1945 Act provided as follows: “Section 7. Changes Made by Federal Government.— (a) If the amount of the net income, as returned by any corporation to the Federal Government, is finally changed or corrected by the Commissioner of Internal Revenue or by any other agency or court of the United States, such corporation, within thirty (30) days after the receipt of such final change or correction, shall make a corrected report, under oath or affirmation, to the department showing such finally changed or corrected net income, upon which the tax is required to be paid to the United States.”
This Section was amended in 1947 by adding subsection: “(d) The provisions of this section shall not be construed so as to permit a resettlement based upon the allowance of any deduction on account of net operating losses, sustained in other fiscal or calendar years, that are not allowed as deductions under the definition of ‘Net Income’, as contained in section two of this act.” The Act of 1947 then provided that it should become effective immediately upon its final enactment, viz., May 14, 1947.
The effect of the amendments in the Act of 1947 are these: No resettlement which was based upon the allowance of any deduction for operating losses sustained in 1944 or 1945 or 1946 could be made after May 14, 1947.
The Act of 1947 means, and it was so interpreted by the fiscal and administrative officers of the Commonwealth, as the Chancellor found, based upon adequate evidence, that in case corporations sustained net operating losses in 1946 and because of going out of business or because of a merger or consolidation or a *170withdrawal from the State or for any other reason, obtained from the Commonwealth a resettlement of its taxes prior to May 14, 1947, that resettlement, with its deduction for a carry-back loss sustained in 1948, was valid; but other corporations of the same class, with net operating losses sustained in 1946, were not entitled to a deduction therefor if no resettlement had been made prior to May 14, 1947. In other words, corporations of the same class, each having net operating losses in 1946, are treated radically differently, depending solely upon the date of resettlement. The liability or non-liability for (a higher) tax was made to depend, not upon the amount of net income as ascertained by the Federal Government, not upon a corporation’s net operating loss in 1946, not upon a justifiable classification of corporations, but solely upon whether a resettlement of carry-back, losses for 1946 and. prior years- was made by the fiscal or administrative officers of. the Commonwealth prior or subsequent to May 14,-1947. • Such a classification,, such a tax, is arbitrary and unreasonably discriminatory; the tax is not applied with substantial equality of the tax burden to all members of the same class. The effect of the Act is to produce arbitrary, unjust and unreasonably discriminatory results and, for these reasons, it violates, so far as these taxpayers are concerned, Article IX, §1 of the' Pennsylvania Constitution and §1 of the Fourteenth Amendment of- the'. Constitution of the United States. - /• ■
The Commonwealth further contends that even if the’Act: of. 1943 ..permitted a deduction from_1944 net income, .of carry-back losses suffered in. 1946, ,tho legislature can change and .prohibit this in. 1947; . We find no. merit ‘in. this:. Contention. ... -,. ■ . . .
■ An Act'of .Congress or .an Act. of .the legislature of Pennsylvania'which imposes- a retroactive tax is . not *171■unconstitutional per se merely because it is retroactive in its language or in its operation: Billings v. United States, 232 U. S. 261, 282; Milliken v. United States, 283 U. S. 15, 21; Welch v. Henry, 305 U. S. 134; Shirks Motor Express Corp. v. Messner, 375 Pa. 450, 462, 100 A. 2d 913.
While a principle more favorable to the taxpayer has been applied in gift tax cases (See Untermyer v. Anderson, 276 U. S. 440, and Nichols v. Coolidge, 274 U. S. 531), laws or acts imposing income taxes or excise taxes or property taxes have been sustained even though applied retroactively during the year of the session in which the taxing statute is enacted, and in some instances during the year of the preceding session. These statutes were sustained even though they increased the tax burden by imposing new taxes or increasing the rates of old ones, or both; and the retroactive effect ivas held not to constitute a denial of due process under either the United States or the Pennsylvania Constitutions: Shirks Motor Express Corp. v. Messner, 375 Pa., supra; Welch v. Henry, 305 U. S., supra; Stockdale v. The Insurance Companies, 20 Wall. 323. It is obvious, hoAvever, that there must be some limitation on the right of a legislative body to pass laAVS imposing taxes retroactively, for otherwise a legislature could constitutionally impose new or increased taxes retroactively for a period of 25 or 50 years Avhich would be so onerous or confiscatory and unjust as to bankrupt the individuals or corporations thus taxed. This Avas recognized in Welch v. Henry, 305 U. S., supra, where the Court sustained an Act of Wisconsin passed in 1935 which for the first time taxed corporate dividends received by taxpayer in the year 1933, a return of Avhich was not due, hoAvever, under Wisconsin law, until March 15, 1934. Justice Stone said (pages 149, 150) : . . The contention that *172the retroactive application of the Revenue Acts is a denial of the due process guaranteed by the Fifth Amendment has been uniformly rejected. Stockdale v. Insurance Companies, 20 Wall. 323, 331; Railroad Co. v. Rose, 95 U. S. 78, 80; Flint v. Stone Tracy Co., 220 U. S. 107; Billings v. United States, 232 U. S. 261, 282; Brushaber v. Union Pacific R. Co., 240 U. S. 1, 20; Lynch v. Hornby, 247 U. S. 339, 343; LaBelle Iron Works v. United States, 256 U. S. 377. . . . And we think that the ‘recent transactions’ to which this Court has declared a tax law may be retroactively applied, Cooper v. United States, 280 U. S. 409, 411, must be taken to include the receipt of income during the year of the legislative session preceding that of its enactment.”
Following Welch v. Henry, we decide that a tax may not be retroactively applied beyond the year of the general legislative session immediately preceding that of its enactment; to provide otherwise constitutes a denial of due process. For this additional reason the Act of 1947 as applied in this case to 1944 corporate net income denies to each of the appellants Due Process and is void.
The case of Commonwealth v. Chambersburg Engineering Co., 287 Pa. 54, 134 A. 408, is distinguishable because, inter alia, the tax base there was the net income as returned to — not ascertained by — the Federal Government. In the instant case neither the Commonwealth nor either of these defendants could know the net income which was its taxable base for the year 1944 until the Federal Government had finally ascertained taxpayer’s 1944 net income after deducting net operating losses incurred in 1946. The legislature obviously realized this and even provided in §7 for a report of change in net income and for a resettlement of the tax where changes occurred and did not, as the *173Commonwealth contends, limit this to fraud or mistake.
The Commonwealth’s further contention that the Acts of 1943 and of 1945 as thus interpreted violate the Uniformity Clause, Article IX, §1 of the Constitution of Pennsylvania is without merit — the rate of tax is the same and the tax base is the same for every corporation of the same class, and therefore there is no lack of uniformity: Commonwealth v. Warner Bros. Theatres, Inc., 345 Pa., supra; Turco Paint & Varnish Co. v. Kalodner, 320 Pa. 421, 184 A. 37.
We deem it unnecessary to discuss the other contentions made by appellant or by appellees.
The judgment in each case is affirmed.
Mr. Chief Justice Horace Stern and Mr. Justice Arnold concur in the result. Mr. Justice Chidsey took no part in the consideration or decision of these cases.Italics throughout, ours.