The hydrolysis of pesticides that are sorbed to sterilized natural sediments has been investigated in aqueous systems at acid, neutral and alkaline pH's. The results show that the rate constants of pH independent ('neutral') hydrolyses are the same within experimental uncertainties as the corresponding rate constants for dissolved aqueous phase pesticides. Base-catalyzed rates, on the other hand, are substantially retarded by sorption and acid-catalyzed rates are substantially enhanced. A large body of evidence will be presented that substantiates these conclusions for a variety of pesticide types sorbed to several well-characterized sediments. The significance of the results for the evaluation of the effects of sorption on the degradation of pesticides in waste treatment systems and natural water bodies also will be discussed.