A process is provided for the enrichment in R isomer of an aryloxypropioic acid ester and for the stereoselective hydrolysis of an aryloxypropionic acid ester to form the corresponding aryloxypropionic acid in predominantly S configuration which comprises subjecting an aryloxypropionic acid ester of formula I ##STR1## wherein A is an ester residue and preferably an alkyl group, optionally substituted or branched, wherein R' is an optionally substituted aryl or heterocyclic ring system, containing in addition to carbon atoms one or more atoms selected from nitrogen, sulphur and oxygen, this ring system being optionally substituted and wherein B and D are for example a hydrogen or a halogen atom, to the action of a microorganism or substance derived thereof capable of stereoselective hydrolysis of the COOA group to form the corresponding carboxylic acid predominantly is S configuration.