Process for preparing O,O-dimethyldithiophosphoric acid and alkaline salts thereof

Process for the continuous production of a stable alkaline solution of O,O-dimethyldithiophosphoric acid of the formula ##STR2## in which Me is Na, K or NH.sub.4, in high yield and of a high degree of purity, and while avoiding explosions, said process consisting in reacting highly reactive phosphorus pentasulphide with methanol in an excess of 30% to 70% by weight of toluene with regard to the reacting mass, at a pressure of from 1 to 5 atm., at a temperature of from 60.degree. to 75.degree. C., in the presence of from about 0.01 to about 0.5% by weight, on the methanol weight, of pyridine or of a pyridine base, for an average reaction time of 30 to 60 minutes, and thereafter treating the toluene solution of O,O-dimethyldithiophosphoric acid thus obtained with an aqueous solution of an alkaline hydroxide.