Production of chlorpyrifos via the phase-transfer-catalyzed reaction of O,O-diethylphosphorochloridothioate and the sodium salt of 3,5,6-trichloropyridin-2-ol was reinvestigated. The formation of sulfotep (the major byproduct) and the yield are influenced by the nature and concentration of the catalysts, temperature, stirring rate, and time of the reaction. The elucidation of the roles of different parameters influencing the end of the reaction have permitted us to perform the synthesis of chlorpyrifos on bench scale (0.3 M scale) under optimized conditionssusing the minimum amounts of catalysts (0.5 mol %)swith 92% yield and 98.5% purity.