PURPOSE: To produce a corresponding saturated mono and di-carboxylic acid in high yield by oxidation reaction of an unsaturated carboxylic acid.

CONSTITUTION: An unsaturated carboxylic acid selected from oleic acid, erucic acid, linolic acid, linolenic acid or ricinolic acid and catalysts of (A) an acid selected from tungstic acid, tungstophosphoric acid, molybdic acid and molybdophosphoric acid and (B) quaternary amine salt are separately charged into a reaction system and the above-mentioned unsaturated carboxylic acid are oxidized by hydrogen peroxide to produce a corresponding saturated mono- and dicarboxylic acid.