The present invention provides a method for making O,S-dimethyl phosphoramidothioate. In accordance with this process, O,O-dimethyl phosphoramidothioate is isomerized in the presence of dimethyl sulfate to form a reaction mixture; the reaction mixture is then passed through a wiped film evaporator at a temperature of from about 55.degree. C. to about 120.degree. C. and a pressure of from about 3 to about 10 mm Hg, with the distillate containing O,S-dimethyl phosphoramidothioate, unreacted O,O-dimethyl phosphoramidothioate and by-products, and the residue containing O,S-dimethyl phosphoramidothioate in an amount greater than the amount of O,S-dimethyl phosphoramidothioate contained in the reaction mixture prior to distillation.