Abstract: The present invention relates to a process for preparing hydroxycarboxylic acids, preferably ?- and ?-hydroxycarboxylic acids, from ammonium carboxylates of the general formula in which R1, R2 and R3 are each independently H, OH, (C1-C6)-alkyl optionally substituted by a hydroxyl group, (C1-C6)-alkenyl optionally substituted by a hydroxyl group, (C1-C6)-alkoxy optionally substituted by a hydroxyl group, (C1-C6) -alkylthio-(C1-C6)-alkyl optionally substituted by a hydroxyl group, (C6-C10)-aryl optionally substituted by a hydroxyl group, (C7-C12)-aralkyl optionally substituted by a hydroxyl group, (C3-C5)-heteroaryl optionally substituted by a hydroxyl group, with the proviso that at least one hydroxyl group is present in at least one R1, R2 and R3 radical, preferably R1?H, CH3, CH2CH3, C6H5, (CH2)2SCH3 and R2?H, CH3 and R3?OH, equally preferably R1?CH2OH, CHOHCH3 and R2?R3?H, CH3, more preferably R1?R2?CH3 and R3?OH, equally more preferably R1?CH2OH, R2?CH3 and R3?H, comprising the following step: heatin