Abstract: The invention relates to the preparation of optically active aryloxyalkanoic acid derivatives of the formula ##STR1## in which Ar represents an optionally substituted phenyl, naphthyl, phenoxy or pyridyloxy radical, R represents a C.sub.1 -C.sub.4 -alkyl radical and A represents a --COOR.sub.1, ##STR2## or --CO--Z radical, Z being halogen. It consists in reacting an aqueous or aqueous-organic solution of an optically active alkali metal salt of a 2-halogeno-alkanoic acid with an aqueous solution of an alkali metal salt of the phenol of the formula Ar-OH at an elevated temperature under a pressure less than the vapor pressure of water at the chosen temperature.It permits the preparation of aryloxyalkanoic acid derivatives with a very high content of the isomer having the D configuration, which can be used as herbicides.