O,O-dialkyl pyrimidylphosphates are made by reacting an O,O-dialkyl phosphorohalothioate with a 5-hydroxypyrimidine in the presence of an imidazole phase transfer catalyst, an aromatic solvent and an alkali metal hydroxide at a pH of from about 8 to about 12 and a temperature of up to about 70.degree. C. The products of this process are obtained in high yield. These compounds are useful as pesticides and insecticides.