Process for the preparation of alkali metal salts of phosphonic acid monomethyl esters

A process for the preparation of an alkali metal salt of a phosphonic acid monomethyl ester of the formula I ##STR5## in which R is C.sub.1 -C.sub.14 -alkyl, and

• Me is sodium or potassium,
• which comprises:
• providing an aqueous reaction medium by metering of 20 to 50% strength aqueous alkali metal hydroxide solution, in a molar ratio of 1:1, into a phosphonic acid dimethyl ester starting material of the formula II ##STR6## said metering being regulated such that the pH of the aqueous reaction medium does not exceed 10, said phosphonic acid dimethyl ester starting material being optionally dissolved in water, the total water content of said aqueous reaction medium being not more than 100% by weight, based on the weight of alkali metal hydroxide and phosphonic acid dimethyl ester starting material, said reaction medium being maintained at a temperature in the range of 90.degree. to 130.degree. C.;
• and
• distilling off methanol produced by the resulting reaction between the aqueous alkali metal hydroxide solution and the phosphonic acid dimethyl ester starting material.