A new empirical electronic descriptor, obtained from a molecular orbital calculation and applicable to benzene derivatives, is proposed. It is shown that this descriptor, the frontier orbital phase angle, correlates very strongly with the pharmacological activity in humans of a large series of hallucinogenic phenethylamines. In the largest QSAR study on such hallucinogens yet reported, it is demonstrated that the phase of mixing of degenerate frontier orbitals of benzene to form the frontier orbitals of the drug results in the best electronic descriptor yet found for hallucinogenic activity in phenylalkylamines.