In a dispute involving the validity of Treasury Regulation section 1.6015-5(b)(1), which establishes a two-year limitations period within which to request equitable innocent spouse relief from joint and several income tax liability, judgment of the district court is reversed where: 1) I.R.C section 6015 is ambiguous on the subject limitations period; and 2) the two-year period within which to request relief under section 6015(f), constitutes a permissible interpretation of the statute under the analytical framework established by Chevron U.S.A., Inc. v. Natural Resources Defense Council, Inc., 467 U.S. 837 (1984).