Oligomerisation of isoprene catalysed by nickel naphthenate and isoprene magnesium was studied under various conditions. The use of various phosphites as donors gives cyclic dimers containing dimethylcyclooctadiene (about 80%); in particular 1, 1, 1-trishydroxymethyipropane phosphite gives trimethylcyclododecatriene selectively. At 500°C and under atmospheric pressure, 1, 5-dimethyl-1-cyclooctene thermally isomerized to 2, 6-dimethyl-1, 7-octadiene. Trimethylcyclododecanone having weak musk-like odour was prepared from trimethyl-1, 5, 9-cyclododecatriene.