Abstract: An improved process for the production of dialkoxy phosphoric acid esters of substituted hydroxypyrimidines which involves reacting a dialkyl phosphoric acid halide with a substituted hydroxypyrimidine at elevated temperatures and in the presence of an inert non-polar organic solvent and an acid binding agent but without catalyst, wherein the phosphoric acid halide and sodium or potassium hydroxide in molar excess of about 5 to 10% are added to a reaction mixture in reflux consisting essentially of the hydroxypyrimidine in molar excess of about 1 to 5% and the non-polar solvent at uniform rates over a period of 1 to 7 hours while removing water substantially as it is formed thereby impeding formation of cholinesterase inhibiting impurities. A preferred embodiment is the preparation of Diazinon.