The SCF-AO's expanded by Löwdin have been used for the calculation of the electron spin dipolar interactions in the triplet-state zero-field splittings of some aromatic molecules. Within a framework of two-center approximation for the spin–spin interaction integrals over atomic orbitals, and adopting the Sklar-type approximation for the exchange-type integrals, excellent agreement with experiments has been obtained for benzene and naphthalene without invoking σ–π interactions. The wavefunctions used were obtained by means of the Pariser–Parr–Pople method, within a full single excitation configuration treatment. This approach has been extended to some aromatic nitrogen heterocycles, but the results, although promising, are poorer, possibly due to the neglect of spin–orbit coupling. Uncertainty in the choice of the C–N resonance integral and the neglect of penetration integrals in the core Hamiltonian may also be factors. Eleven molecules have been treated in all. For comparison, the single-determinant unrestricted Hartree–Fock wavefunctions also have been employed, but these give extremely poor agreement with experiment.