Cite as: 540 U. S. 461 (2004)
Opinion of the Court
The CAA construction EPA advances in this litigation is reflected in interpretive guides the Agency has several times published. See App. 268-269 (1983 EPA PSD guidance memorandum noting the Agency's "oversight function"); id., at 274 (1988 EPA guidance memorandum stating EPA may find a BACT determination deficient if it is "not based on a reasoned analysis"); id., at 281-282 (1993 guidance memorandum stating that "EPA acts to ensure that the state exercises its discretion within the bounds of the law" (internal quotation marks omitted); as to BACT, EPA will not intervene if the state agency has given "a reasoned justification for the basis of its decision" (internal quotation marks omitted)). See also Approval and Promulgation of Air Quality Implementation Plans; Commonwealth of Virginia—Prevention of Significant Deterioration Program, 63 Fed. Reg. 13797 (1998) (EPA will "review whether any determination by the permitting authority was made on reasonable grounds properly supported on the record, described in enforceable terms, and consistent with all applicable requirements"). We "normally accord particular deference to an agency interpretation of 'longstanding' duration," Barnhart v. Walton, 535 U. S. 212, 220 (2002) (quoting North Haven Bd. of Ed. v. Bell, 456 U. S. 512, 522, n. 12 (1982)), recognizing that "well-reasoned views" of an expert administrator rest on " 'a body of experience and informed judgment to which courts and litigants may properly resort for guidance,' " Bragdon v. Abbott, 524 U. S. 624, 642 (1998) (quoting Skidmore v. Swift & Co., 323 U. S. 134, 139-140 (1944)).
We have previously accorded dispositive effect to EPA's interpretation of an ambiguous CAA provision. See Chevron U. S. A. Inc. v. Natural Resources Defense Council, Inc., 467 U. S. 837, 865-866 (1984); Union Elec., 427 U. S., at 256. The Agency's interpretation in this case, presented in internal guidance memoranda, however, does not qualify for the dispositive force described in Chevron. See Christensen v. Harris County, 529 U. S. 576, 587 (2000) ("Interpretations
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