Abstract: Process for the preparation of dienes and/or trienes comprising reacting a polyolefinically unsaturated compound in the presence of a heterogeneous disproportionation catalyst with a compound according to the general formula ##STR1## wherein R.sup.1 and R.sup.2, which may be the same or different, each represents an alkyl, aryl, alkaryl or aralkyl group which may or may not contain one or more inert substituents or R.sup.1 and R.sup.2 form part of a cyclic structure together with the carbon atom to which they are attached. Isononadiene (2,6-dimethyl-1,5-heptadiene) and/or 2,6,10-trimethyl-1,5,9-undecatriene can be obtained by reacting cis-1,4-polyisoprene with isobutene in the presence of a heterogeneous disproportionation catalyst.