The oxidation rate of chlorpyrifos was examined in the pH range of 6-11 in the presence of free chlorine. As pH decreased, the observed rate of chlorpyrifos oxidation increased. Hypochlorous acid appears to be the primary oxidant responsible for chlorpyrifos oxidation. The reaction order with respect to both free chlorine and chlorpyrifos was found to be first order. A simple model was developed to predict the apparent second order rate coefficient over the pH range examined. The objectives of this study were to develop experimental approaches and a modeling philosophy to study degradation of organophosphate pesticides as a class under drinking water treatment conditions. Includes 26 references, figures.