The present invention relates to a process for preparing hydroxycarboxylic acids, preferably α- and β-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: heating an aqueous starting solution comprising the ammonium carboxylate to form, by thermal decomposition of the ammonium carboxylate, the hydroxycarboxylic acid and ammonia, and simultaneously to remove at least a portion of the free water and of the ammonia formed from the solution and thus to obtain a product fraction comprising the hydroxycarboxylic acid, characterized in that the content of the ammonium salt in the starting solution is less than 60% by weight, the thermal decomposition of the ammonium salt and the removal of the free water and of the ammonia formed are effected in one process step, the conversion of the ammonium salt being more than 20 mol %, preferably more than 30 mol %, more preferably more than 50 mol %, especially preferably more than 75 mol %, very especially preferably more than 90 mol % and especially more than 95 mol %, and no ether, alcohol or hydrocarbon is used as an entraining agent.