The kinetics of hydrolysis of selected organophosphorothioate insecticides were determined in sediment-water samples to define the role of hydrolysis in the sediment-sorbed state. Investigations with well-characterized sediments showed that the rates of neutral hydrolysis of chlorpyrifos (0,0-diethyl 0-(3,5,6-trichloro-2-pyridyl) phosphorothioate), diazinon (0,0-diethyl 0-(2-isopropyl-4-methyl-6-pyrimidyl) phosphorothioate), and Ronnel (0,0-dimethyl 0-(2,4,5-trichlorophenyl) phosphorothioate) are unaltered when the compounds are sorbed to sediments. For chlorpyrifos, the observed rate constants were the same in the sediment and aqueous phases and similar in magnitude to those found for natural water samples. Limited data for diazinon and Ronnel indicated similar kinetic behavior for these related compounds. Alkaline hydrolysis rates in the sediment-sorbed phase for chlorpyrifos, on the other hand, were slowed considerably relative to those in the aqueous phase. Experiments indicated that the rate of alkaline hydrolysis in the sorbed state is approximately 10 times slower than in the bulk solution.