PURPOSE: To easily obtain phosphonoorganocarboxylic acids at a high yield by treating a mixture obtd. by heating an unsatd. carboxylic acid, etc., together with a phosphorus chloride reactant by water.

CONSTITUTION: The compd. (e.g.; maleic acid, etc.) of formula (R1 is H, 1 to 6C alkyl; R2 is H, 1 to 6C alkyl, 6 to 18C aryl, etc.) or its salt (alkaline metal salt, etc.), or its anhydride (e.g.; acrylic acid anhydride, etc.) is heated to 80 to 120°C under anhydrous and inert gases together the phosphorus chloride reactant [preferably the reactant of phosphorus trichloride and active hydrogen- contg. compd. and the compd. of formula R3OH (R3 is 1 to 6C alkyl, etc.), etc.], and is brought into reaction for 0.5 to 10 hours and is then hydrolyzed in water, aq. acid, alkaline water, etc., by which the objective compd. is obtd.