1,5-Dimethyl-1,5-cyclooctadiene and 1,4-dimethyl-4-vinyl-1-cyclohexene are prepared by contacting isoprene with a catalyst obtained by mixing a nickel compound, a reducing agent capable of reducing the nickel to zerovalent nickel, and a trihydrocarbyl phosphite, arsenite or antimonite wherein at least one of the three hydrocarbyl groups is a substituted hydrocarbyl group CF.sub.3 --(CF.sub.2).sub.n C(H)(R)--, in which n.gtoreq.0 and R is an optionally substituted hydrocarbyl group. Only very small amounts of the undesired 1,6-dimethyl-1,5-cyclooctadiene are formed.