Abstract: Olefins are polymerized using, as a catalyst, a pi-olefin complex of a transition metal supported on aluminophosphate. The most effective complexes are cycloheptatrienyl-(1,3-cycloheptadiene)chromium(-1) compounds and alkyl-substituted derivatives thereof. A metal alkyl co-catalyst, such as triethylborane or triethylaluminum, can also be present. The catalyst is best suited for forming ethylene polymers and copolymers. Both the polymerization process using the described catalyst, and the catalyst system, per se, constitute different aspects of the invention.