The vinylimmonium cation [CH2[DOUBLE BOND]CHCHNH2]+, [1H]+, originating from cyclopentylamine has been studied by photoionization, ion cyclotron resonance mass spectrometry and ab initio molecular orbital calculations. A heat of formation, ΔHmath image[CH2[DOUBLE BOND]CHCHNH2]+ of 747±7 kJ/mol has been deduced from photoionization experiments, after correction for the thermal energy of the precursor molecules. The proton affinity of vinylimine, CH2 [DOUBLE BOND] CHCHNH, 1, (PA(1) = 908±4kJ/mol) has been deduced from proton exchange experiments using the ‘bracketing’ technique. This latter, combined with the previously determined ΔHmath image[CH2[DOUBLE BOND]CHCHNH2]+ leads to ΔHmath image(CH2[DOUBLE BOND]CHCHNH)=125kJ/mol with a possible error of ±11 kJ/mol. Molecular orbital calculations, conducted at the MP2/6-311 ++G**//MP2/6-31G* + ZPE level, lead to comparable estimates ΔHmath image(CH2 [DOUBLE BOND] CHCHNH2)+ = 746kJ/mol and ΔHmath image(CH2[DOUBLE BOND]CHCHNH) = 128–135 (kJ/mol).