Chlorpyrifos (CP) was used as a model compound to develop experimental methods and prototype modeling tools to forecast the fate of organophosphate (OP) pesticides under drinking water treatment conditions. CP was found to rapidly oxidize to chlorpyrifos oxon (CPO) in the presence of free chlorine. The primary oxidant is hypochlorous acid (HOCl); thus, oxidation is more rapid at lower pH. At elevated pH, both CP and CPO are susceptible to alkaline hydrolysis and degrade to 3,5,6-trichloro-2-pyridinol (TCP), a stable end product. Furthermore, the hydrolysis of both CP and CPO to TCP was shown to be accelerated in the presence of free chlorine. These observations regarding oxidation and hydrolysis are relevant to common drinking water treatment processes: disinfection and water softening, respectively. In this work, intrinsic rate constants for these processes were determined, and simple computer models have been developed that accurately predict the concentration of CP, CPO, and TCP as a function of pH, chlorine dose, CP concentration, and time after chlorine dosing. These models serve as a first step toward the development of tools to assess the assessment of risk associated with consuming treated drinking water whose source is contaminated with OP pesticides.