The present invention relates to a method for isomerizing a 3-(Z)-unsaturated carboxylic acid of the formula l-Z or a salt thereof, wherein R 2 is C 1 -C 24 -alkyl, C 2 -C 24 -alkenylhaving 1, 2, 3 or more than 3 C-C double bonds, unsubstituted or substituted C 5 -C 12 -cycloalkyl, or unsubstituted or substituted aryl; R 1 is hydrogen or has one of the definitions specified for R 2 ; with the proviso that R 2 has a higher priority than R 1 in accordance with lUPAC; to give a 3-( E )-unsaturated carboxylic acid of the formula I-E or a salt thereof, wherein the isomerization of the compound of the formula l-Z is effected in the presence of an anhydride of an organic acid and a base or in the presence of a ketene of formula CR 11 R 12 C(0), wherein R 11 and R 12 are as defined in the claims and in the specification and a base. In particular, the present invention relates to a method for preparing compositions with increased content of (3 E , 7 E )-homofarnesylic acid starting from compositions comprising (3 Z ,7 E )- and (3 E , 7 E )-homofarnesylic acid.