A two step process for producing isobutylene from propylene and synthesis gas. In the first step, propylene is contacted with synthesis gas at elevated temperature in the presence of a catalyst system containing a mixture of monometal compounds M.sub.4 (CO).sub.12 and M'.sub.4 (CO).sub.12 and/or bimetal compound M.sub.2 M'.sub.2 (CO).sub.12 where M and M' are different metals of the cobalt group. The alcohol mixture thus-obtained is dehydrated in the second step, to produce a product fraction containing a large amount of isobutylene.