The energies of 166 molecules, radicals, anions, and cations have been calculated using four recently developed model chemistries : G2, G2(MP2), CBS-4, and CBS-Q. The results of these calculations are compared with the experimental heats of formation, bond dissociation energies, gas phase acidities, and proton affinities. Both G2 and CBS-Q reproduce the known heats of formation of molecules with an average error of only 1 kcal/mol. The bond dissociation energies (BDE) derived from G2, CBS-Q, and CBS-4 also agree with the experimental data with an average error of only 1 kcal/mol. The BDE for several series (HX, CH(3)X, CH2=CHX, HC=CX, and CH(3)COX, where X is a first- or second-row substituent) were compared, and it was found that the coordinately unsaturated substituents (Li, BeH, BH2, Na, MgH, and AIH(2)) gave essentially the same effect in each series. Differences were found with the substituents having lone pairs (NH2, OR, F, PH2 SH, and Cl). It was possible to correlate a wide variety of bond dissociation energies using a Pauling-type relationship based on electronegativity differences. Gas phase acidities and proton affinities also were examined, and were equally well reproduced.