Previous studies have shown that impurities in commercial organophosphorus insecticides induce a variety of toxicological manifestations. Few studies have contrasted common impurity types and their comparative chemical and biochemical properties. In this study, five O,O-dimethyl phosphorothioate compounds were converted to their corresponding O,S-dimethyl phosphorothioates (isomerides) by a stepwise dealkylation-alkylation process (yields 58-76%). The O,S-isomerides and parent material were characterized by reverse-phase high-performance liquid chromatography (RPHPLC) and phosphorus (31P) nuclear magnetic resonance (NMR) spectroscopy. Methanol-water mixtures were found to adequately separate isomeride from parent structure with the isomeride eluting first. In general, the O,S-isomerides were found to be shifted about 40 ppm upfield relative to the O,O material. Isomerides were also determined to be significantly more potent as anticholinesterases (rat brain), with ki values approximating 1000-fold those of the parent material.