The m/z 44 appearance energies for five primary amines have been measured by threshold photoionization mass spectrometry. Following an analysis of the thermochemistry associated with these unimolecular fragmentations, a value of 665.1 +/- 1.4 kJ mol(-1) is obtained for the 298 K heat of formation for the ethylidenimmonium cation (CH3CH=NH2+). When combined with high-level ab initio calculations, this results in absolute proton affinities of 906.4 +/- 2.7 and 909.2 +/- 2.8 kJ mol(-1) for the ethylidenimines E-CH3CH=NH and Z-CH3CH=NH, respectively.