Heats of formation of aliphatic imines and immonium ions calculated with the G2(MP2) and CBS-Q composite ab initio methods are in reasonable agreement with experimentally determined literature values, particularly when the 298 K heats of formation of immonium ions are rederived from threshold energy measurements with inclusion of the 298 K enthalpy contributions proposed by Traeger and McLoughlin, The heats of formation calculated with the simplified procedures, G2(MP2,SVP) and CBS-q, can deviate significantly from the G2(MP2) and CBS-Q results, whereas the four methods yield very similar proton affinities. The increase in proton affinity brought about by the introduction of substituents at the imine carbon is substantial, masking any effects of the nitrogen atom hybridization. Cis imines are more basic than their trans isomers, which reflects that the energy difference between cis and trans isomers of imines is unusually high, 15-17 kJ mol(-1). Protonation of imines changes the energy difference between the favored, eclipsed conformers. A Benson-type additivity scheme for the heats of formation of immonium ions is proposed, and new group additivity values for imines are determined.