The heats of formation of molecular ions are often not known to better than 10 or 20kJ/mol. The present study on nitrogenous compounds adopts the graphical approach of Holmes and Lossing [8] which relates cation heats of formation to cation size. A study of methyl substitution in formamides and acetamides is followed by an examination of heat of formation data on carbon-site and nitrogen-site methyl substitution in immonium, amine, imine, ammonium and amino cations. The results provide tests of the validity of this graphical method and also suggest investigating or re-investigating the ionization energies and the heats of formation of several of the molecules studied.