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0.5 % CO, 1.5 % CH4 (by mass), and N2 in an intimate mixture (Owen et al. 1993). Assuming the N2–CO–CH4 frost acts like an ideal mixture,We obtained Pluto’s spectrum using the CSHELL echelle one finds that XCH4 is only 0.0001 % to 0.001%. However, Lellouch (1994)spectrograph at NASA’s IRTF on Mauna Kea, on 25–26 May points out that Triton’s XCH4 is several hundred times greater than that1992, with a spectral resolution of 13,300. The spectral range predicted by the ideal mixture that Cruikshank et al. (1993) used to (5998–6018 cm–1, or 1661.8–1666.9 nm) includes the R(0) and interpret similar spectra of Triton. This can be explained if the N2–CO– the Q(1)–Q(9) lines of the 2n3 band of methane. The resulting CH4 frost is nonideal. Lellouch therefore suggests that Pluto’s XCH4 is spectrum shows the first detection of gaseous methane on Pluto, inflated by a similar amount, so that XCH4 P 0.1%. Finally, there is recent with a column height of 1.2013.1520.87 cm-A (3.2218.4622.34 3 1019 molecule spectral evidence of pure CH4 on Pluto’s surface (Schmitt et al., 1994). Pure CH4 frost would supply more gaseous CH4 than N2–CO–CH4 frost,cm–2). 1997 Academic Press raising XCH4 above the ideal-mixture value. The thermal structure of Pluto’s atmosphere is further evidence that Introduction. This paper presents the first positive detection of gas