Abstract: This invention relates to a process for preparing monocarboxylic or dicarboxylic acids by means of an oxidative scission of vicinal diols with H.sub.2 O.sub.2. One starts from vicinal, either terminal or internal, diols soluble in water and, if desired, carrying functional groups that are inert under the reaction conditions. The vicinal diols are reacted in an aqueous solution with H.sub.2 O.sub.2 at a temperature between 0.degree. and 120.degree. C., and at a pH value between 0.5 and 4, in the presence of a catalytic system consisting of tungstic acid or an alkali metal tungstate, and possibly of phosphoric or arsenic acid or an alkali metal salt thereof.