Ab initio calculations in a gaussian orbital basis (LCGO) are reported for naphthalene, indene, styrene, indole, benzofuran, and benzothiophen. A bond energy analysis on small non-aromatic molecules yields energies for the classical Kekulé structures; the resonance energies derived for the real molecules, in close agreement with figures based upon thermochemical data, are: naphthalene 357, indene 225, styrene 208, indole 308, benzofuran 232, benzothiophen 183, benzene 212 kJ mol–1. The calculated dipole moments are in reasonable agreement with experiment, and are separated into σ- and π-components. The high values of the π-moments, and the degree of scatter of the π-electrons into isolated pairs are related to aromatic character. Although the molecules are not isoelectronic, extensive correlation of the orbitals in the series can be achieved, and the HeI photoelectron spectra are interpreted in terms of the main groups of orbital energies.