Phenthoate [O,O‐dimethyl S‐(α‐ethoxycarbonyl)benzyl phosphorodithioate] was rapidly hydrolyzed by rat liver and plasma carboxylesterases to the corresponding nontoxic metabolite, phenthoate acid. A partially purified enzyme isolated from rat liver microsomes was sevenfold more effective in hydrolyzing phenthoate than the microsomal fraction. O,S,S‐Trimethyl phosphorodithioate (TMPDT) and O,O,S‐trimethyl phosphorothioate (TMPT), two impurities present in technical formulations of phenthoate, were examined for their inhibiting effects on the esterase degradation of [phenyl‐14C] phenthoate in vitro. Incubation of [14C]phenthoate with rat liver and plasma carboxylesterases in the presence of these impurities greatly diminished the amount of phenthoate acid formed. TMPDT was superior in its inhibitory action against rat liver carboxylesterase to that of TMPT. TMPDT was equipotent in inhibiting crude rat liver and plasma carboxylesterases, and TMPT was more effective in inhibiting plasma carboxylesterase than rat liver carboxylesterases.