Fourier transform ion cyclotron resonance experiments have been carried out to probe the nature of the potential energy surface for the reaction of methyl cation (CH 3 +) with methane (CH 4) to give the ethyl cation (C 2H 5 +). Product distributions for reactions of CH 3 + and CD 3 + with CD 4 and CH 4, respectively, are found to give a near statistical distribution of ethyl cation products, in good agreement with previous work. When the methyl cation is initially coordinated to HF in the form of a methylfluoronium ion, however, the product distributions are decidedly nonstatistical and are indicative of a reaction which is very nearly thermoneutral. Thermochemical data for relevant species suggest that the reaction is very slightly endothermic. All of the experiments support the intermediacy of a C 2H 7 + complex in the reactions. Ab initio calculations, in conjunction with all of the experimental data, reveal that there are likely three different stable forms of C 2H 7 + involved in the potential energy surface for the reaction. One of the forms, a nonclassical C 2H 5 + coordinated to H 2 in a proton bound dimerlike structure, has not previously been considered to play a role in this reaction. The existence of this structure is supported by infrared multiphoton dissociation experiments on C 2H 7 + previously carried out by Lee and co-workers (J. Am. Chem. Soc. 111 (1989) 5597) and the high pressure mass spectrometric experiments of Hiraoka and Kebarle (J. Am. Chem. Soc. 98 (1976) 6119).