Process for the production of thiophosphoric acid esters

In a process for the production of thiophosphoric acid esters of the formula I ##STR4## wherein R.sub.1 is lower alkyl, lower alkenyl, lower alkoxy(lower)alkyl or lower alkylmercapto(lower)alkyl, R.sub.2 is hydrogen, lower alkyl or lower alkenyl and R.sub.3 and R.sub.4 are lower alkyl, which comprises reacting a dialkyl phosphoric acid halide of the formula II ##STR5## wherein Hal represents chlorine or bromine and R.sub.3 and R.sub.4 are as defined hereinabove, with a hydroxypyrimidine of the formula III ##STR6## wherein R.sub.1 and R.sub.2 have the significance given to them above, at elevated temperatures and in the presence of an organic solvent and an acid binding agent, the improvement which comprises adding the reactant of formula II and sodium or potassium hydroxide concomitantly to a reaction mixture in reflux consisting essentially of reactant of formula III and an inert non-polar organic solvent at uniform rates over a period of 1 to 7 hours while removing water substantially as it is formed, said reactant of formula III being in a molar excess of about 1 to 5% and said acid binding agent being in a molar excess of about 5 to 10%, and said reaction being carried out in the absence of a catalyst.