The persistence of diazinon and Zinophos® (O, O-diethyl O-2-pyrazinyl phosphorothioate) was determined in autoclaved and nonautoclaved Sultan silt loam at 3 temperatures (15, 25, and 35°C), 4 soil-moisture levels (2, 10,20, and 30%), and 4 pH levels (4.3, 5.5, 7.0, and 8.0). Insecticide residues were extracted with a 2:1 mixture of hexane and acetone and measured by gas chromatography. Degradation rates for diazinon were similar in autoclaved and nonautoclaved soil, while Zinophos degraded faster in nonautoclaved soil than in autoclaved soil. Higher temperatures and soil moisture levels accelerated decomposition of both insecticides. Increasing soil acidity accelerated the nonbiological degradation of diazinon. Zinophos was most persistent at pH 5.5 and least persistent in neutral and alkaline soils. Soil pH had no effect upon the nonbiological decomposition of Zinophos but neutral and alkaline soils apparently provided a suitable environment for microorganisms which metabolized the insecticide. Acidophilic microorganisms may have contributed to the degradation of Zinophos at pH 4.3.