Alkyl- and arylmalonic acids of the formula I ##STR1## where R.sup.1 =H, C.sub.1 -C.sub.12 -alkyl, phenyl, C.sub.1 -C.sub.4 -alkylphenyl, C.sub.2 -C.sub.4 -dialkylphenyl, R.sup.2 =C.sub.1 -C.sub.12 -alkyl, phenyl, C.sub.1 -C.sub.4 -alkylphenyl, C.sub.2 -C.sub.4 -dialkylphenyl or R.sup.1 +R.sup.2 =--CH.sub.2 --CH.sub.2 --, are prepared by alkaline saponification by hydrolyzing the corresponding C.sub.1 -C.sub.4 -alkyl esters of the malonic acid of formula I, with alkali metal hydroxide dissolved in an aqueous alkali metal salt solution containing salt at 90-100% of saturation, acidifying the hydrolysis product with a mineral acid, removing the precipitated alkali metal salt which forms upon acidification, and extracting the alkyl- and arylmalonic acid formed from the aqueous solution with the aid of an organic solvent.