Using an energy-resolved electron beam, the gas-phase heats of formation of [CH2NH2]+, [CH3CHNH2]+, [CH3NHCH2]+, [(CH3)2CNH2]+, [(CH3)2NCH2]+, [CH3CHNHCH3]+, [CH3CH2NHCH2]+, and [CH3CH2CHNH2]+ ions have been measured from their appearance energies in ionic dissociations. All of these ions can have a quaternary immonium structure as one of the canonical forms and are consequently more stable than other isomers. Introduction of the —NH2 group into a CH3+ ion brings about a decrease of over 4 eV in the heterolytic bond dissociation energy D(R+—H−), with slightly larger decreases for —NHR and —NR2 groups.