Abstract: The invention relates to enantiomerically enriched malonic acid monoesters &agr;-monosubstituted by a tertiary hydrocarbon radical, or their salts, of the general formula I: in which R1, R3, R4 and R5 are identical or different hydrocarbon radicals, and wherein any two of the radicals R3, R4 and R5 may alternatively be present as a carbocyclic ring together with the quaternary carbon atom which they substitute, and M is hydrogen, one equivalent of a metal or an optionally substituted ammonium ion. The invention further relates to a process for the preparation of the enantiomerically enriched &agr;-monosubstituted malonic acid monoesters, or their salts, by enzymatic partial hydrolysis of the corresponding diesters.