Abstract It has been shown that Manca's empirical relation between bond energies and band gaps is approximately valid for a wide variety of binary semiconductors and insulators. This relationship has also been theoretically derived for the case of ionic compounds, e.g. alkali halides. The procedure of derivation involves application of the Madelung approach and the Born-Haber cycle. The possible theoretical significance of our previous empirical correlation between band gaps and heats of formation per equivalent has also been indicated.