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 reaction in a two-step reaction is disclosed. In reaction step (a) the hydroxypyrimidine derivative is converted with the aid of sodium hydroxide and in the presence of a hydrocarbon solvent and a phase-transfer catalyst to the sodium salt while drying the reaction mixture by azeotropically removing water. In step (b) the condensation process is carried out at room temperature to about 100.degree. C. The condensation step may be accelerated by addition of 4-bis(C.sub.1 to C.sub.4 alkyl)aminopyridine as catalyst.