Abstract The electronic energies from HF 3–2lg*. HF/6-31gM and HF/6–3 lg* ab initio geometry optimization calculations are used with tabulated thermochemical data to obtain values for the gas-phase heats of formation of the quinoline, benzothiophene and thienopyridine isomers. Our approach is based on ring fusion reactions in which pairs of singlering aromatic molecules are fused with elimination of ethene. and on reactions in which the isolobal fragments CH and N exchange places in the fused two-ring species. For benzo[ b]thiophene and naphthalene our calculated values are about 8 kJ mol −1 greater than the accepted experimental values: it is reasonable to suppose that the accuracy of the whole set of calculations is uniform so our estimates for the species for which experimental data is lacking are probably good to better than 10kJ mol −1. Similar calculations for the tautomers of the three isomers of 4-hydroxythienopyridine correctly predict the preference of the [2,3- b)] isomer for the enol form, and of the [3,2- b] and [3,4- b] isomers for the keto form.