Abstract We studied the photochemical oxidation of an organophosphorus pesticide, terbufos, in surface waters because its oxidation products are of concern. Terbufos sulfoxide and terbufos sulfone have toxicities for aquatic organisms similar to those of the parent compound. We measured a photolysis rate constant under natural sunlight conditions in distilled water of 0.68±0.17/h near Clemson, South Carolina, USA. Actinometer studies ...