The first photoelectron bands of the series of simple alkyl free radicals CH3, CH3CH2, CH3CD2, CH3CHCH3, and C (CH3) 3 have been obtained. The adiabatic and vertical ionization potentials, respectively, are 9.84±0.02 eV for methyl radical, 8.39±0.02 and 8.51±0.02 eV for ethyl radical, 8.38±0.02 and 8.50±0.02 eV for ethyl-/, l-di radical, 7.36±0.02 and 7.69±0.02 eV for isopropyl radical, and 6.70±0.03 and 6.92±0.03 eV for tert-butyl radical. The heats of formation of the corresponding carbonium ions are calculated to be 261.8±0.5 kcal/mol for methylcation, 219.2±1.1 kcal/mol for ethyl cat-ion, 187.3±1.1 kcal/mol for isopropyl cation, and 162.9±1.2 kcal/mol for tert-butyl cation. The implicationsof these data for the gas-phase proton affinity scale are explored. Band structure is resolved, and possible assignments are presented. The results are discussed in terms of the interactionsof methyl groups with trigonal carbon radical and ion centers.