Abstract: 1,3,4-Thiadiazol-5-(4H)-onyl-(4)-methyl dithiophosphoric acid ester of the formula ##STR1## wherein each of R, R.sub.1 and R.sub.2 methyl or ethyl and Y represents oxygen or sulphur, are prepared by condensing a thiophosphate acid of the formula ##STR2## or an alkali metal or ammonium salt thereof, with a 1,3,4-thiadiazole of the formula ##STR3## with formaldehyde in an aqueous medium consisting essentially of 60-98 wt % of sulphuric acid, the amount of sulphuric acid is at least 0.7 mol per mol of free thiophosphoric acid, if free acid is used, and 1.7 mol per mol of thiophosphoric acid salt, if a salt is used. The temperature of the reaction mixture is maintained at 15.degree.-60.degree. C. The process is particularly useful in the preparation of O,O-dimethyl-S-[2-methoxy-1,3,4-thiadiazol-5(4H)-onyl-(4)-methyl]-dithioph osphate (methidathion).