The heats of formation of six radical cations have been calculated using ab initio MO methods at the MP4/6-31 + G(2df, p) level with MP2/6-31G(d, p)-optimized geometries. The theoretical values for DELTA-H(f,298)0 (kJ/mol) of the radical ions considered are (experimental values in parentheses): methanol CH3OH+.: 854 (845); methyleneoxonium CH2OH2+.: 815 (816); methyleneimine CH2NH+.: 1076 (1054); aminomethylene HCNH2+.: 1040 (1079); methylamine CH3NH2+.: 863 (843) and methyleneammonium CH2NH3+.: 855 (958). The calculated results thus confirm the discrepancy between experiment and theory on the heats of formation of nitrogen-containing radical cations. In the latter, the distonic species are calculated to be more stable than their classical isomers. The higher stability of the distonic ions has also been discussed. The recommended heat of formation of the methyleneiminium cation CH2NH2+ is 754 kJ/mol.