Photoionization mass spectrometry has been used to measure appearance energies for immonium cation formation from 25 alkyl amine precursors. A number of the unimolecular fragmentation processes are shown to involve excess energy at threshold so that, of the 11 different cations investigated, it is only possible to derive reliable 298 K heats of formation for CH2NH2+ (749.0 +/- 0.9 kJ mol(-1)), CH3CHNH2+ (666.1 +/-1.1 kJ mol(-1)), C2H5CHNH2+ (636.8 +/- 2.5 kJ mol(-1)), CH2NH(CH3)(+) (706.1 +/- 1.0 kJ mol(-1)), CH2NH(C2H5)(+) (668.4 +/- 1.3 kJ mol(-1)), and CH2N(CH3)(2)(+) (668.0 +/- 2.5 kJ mol(-1)). When these are compared to those calculated by the G3, G3B3, G2, G2(MP2), CBS-APNO, and W1U composite ab initio methods, it is found that the smallest mean absolute deviation of 1.2 +/- 0.8 kJ mol(-1) is obtained from the G2 calculations.