This invention relates to a process for obtaining monocarboxylic and dicarboxylic acids from unsaturated carboxylic acids and/or their derivatives. The said process comprises an oxidative cleavage reaction of vicinal diols into which are fed at least some of the aqueous phase separated out at the end of the reaction itself and at least one base so that the pH of the aqueous solution at the start of the oxidative cleavage reaction is between 4 and 7.