A process for preparing enantiomerically enriched E-(2S) - and (2R)-alkyl-5-halopent-4-enecarboxylic acids or their esters of the formula (Ia) or (Ib) in which R is a C 1 -C 6 -alkyl radical, A is H, R 1 , where R 1 can be C 1 -C 4 -alkyl, or R 2 , where R 2 is an alkyl group but is not identical to R 1 , and X is chlorine, bromine or iodine, in which an enantiomeric mixture of a 2-alkyl-5-halopent-4-enecarboxylic ester of the formula (II) in which R, R 1 and X are defined as above, is reacted with a stereoselective hydrolase, which is obtained from microorganisms or plants, in the presence of water or an alcohol of the formula R 2 OH, where R 2 is an alkyl group which is not identical to R 1 , as nucleophile.