The present invention relates to a process for preparing an olefinically unsaturated phosphonium salt having at least two olefinic double bonds, of which one double bond has a Z or cis conformation and the second or a further double bond an E conformation, from an olefinically unsaturated phosphonium salt having at least two olefinic double bonds of different configuration compared to the target compound, and both especially have an all-E or all-trans conformation. In addition, the invention provides for the use of the compound obtained by the process for provision of terpenoid substances as medicaments.