The present invention relates to a method for isomerizing a 3-(Z)-unsaturated carboxylic acid of the formula 1-Z or a salt thereof, wherein R2 is C1-C24-alkyl, C2-C24-alkenyl having 1, 2, 3 or more than 3 C—C double bonds, unsubstituted or substituted C5-C12-cycloalkyl, or unsubstituted or substituted aryl; R1 is hydrogen or has one of the definitions specified for R2; with the proviso that R2 has a higher priority than R1 in accordance with IUPAC; 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 1-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 CR11R12C(0), wherein R11 and R12 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 (3E,7E)-homofarnesylic acid starting from compositions comprising (3Z,7E)- and (3E,7E)-homofarnesylic acid.