A cis-trans isomerization of a cis-.beta.-.gamma.-unsaturated alcohol or its ester having the formula ##STR1## wherein R.sub.1 represents hydrogen atom or a C.sub.1 -C.sub.5 alkyl group; X represents hydrogen atom or ##STR2## and R.sub.2 represents a hydrocarbon moiety or a hydroxyhydrocarbon moiety; R.sub.3 represents hydrogen atom or a hydrocarbon moiety into a transform, in the presence of a mercaptan as a catalyst, is provided. It can be also attained in the presence of a mercaptan and/or a disulfide as a catalyst in an inert gas environment with an initiator. It can be also attained in the presence of a mercaptan under an irradiation, if desired with a small amount of an initiator.