Preparation of O,O-dimethyl phosphoramidothioate and O,O-dimethyl phosphoroamidothioate. A process of making O,O-dimethyl phosphoroamidothioate is described including reacting sulfur with PCl3 to form PSCl3, reacting the PSCl3 formed with methanol to form O-methyl phosphorodichloridothioate, and reacting the O-methyl phosphorodichloridothioate formed with methyl lye to form O,O-dimethyl phosphorochloridothioate in solution in CH2Cl2, and reacting the O,O-dimethyl phosphorochloridothioate formed with sodium hydroxide and ammonium hydroxide to form O,O-dimethyl phosphoroamidothioate in solution in CH2Cl2. Reacting the O,O-dimethyl phosphoroamidothioate formed with catalytic dimethyl sulfate to form methamidophos and reacting the methamidophos formed with acetic anhydride to form N-(methoxy-methylsulfanylphosphoryl) acetamide is also described. Throughout the process, the O,O-dimethyl phosphorochloridothioate and the O,O-dimethyl phosphoroamidothioate formed are maintained in solution in CH2Cl2 at all times.