A process for preparing 9-(Z)-retinoic acid from mother liquors from the industrial preparation of C.sub.15 -triarylphosphonium salts of the general formula I ##STR1## where R.sup.1 to R.sup.3 are each aryl and X.sup..crclbar. is halogen or (HSO.sub.4).sup..crclbar., in an organic solvent, which comprisesA. increasing the proportion of 9-(Z)-C.sub.15 -triarylphosphonium salt in the C.sub.15 -triarylphosphonium salts isolated from the mother liquor by treatment with isopropanol at elevated temperature, cooling and separating off the all-(E)-C.sub.15 -triarylphosphonium salt which has crystallized out,B. subjecting the resulting C.sub.15 -triarylphosphonium salt to a Wittig reaction with an alkyl .beta.-formylcrotonate of the general formula ##STR2## and C. hydrolyzing the resulting oily retinoic ester mixture in a C.sub.3 -C.sub.9 -alkanol, preferably in a propanol or butanol, precipitating the resulting 9-(Z)-retinoic acid where appropriate by adding methanol as crystals, with all-(E)-retinoic acid and other retinoic acid isomers remaining in the alkanolic solution.