The compounds [Ir(cod)X](2) (cod = 1,5-cyclooctadiene; X = Cl, OMe) catalyze the polymerization of phenylacetylene, with negligible formation of oligomeric products. At variance, the monoolefin analogue [Ir(cot)(2)Cl](2) (cot = cyclooctene) only promotes alkyne cyclotrimerization. The polymerization reaction proceeds in various solvents such as tetrahydrofuran (THF), chloroform and benzene, but it is most favored when using NEt3 as reaction medium. The role of the diene in the catalytic reaction is investigated, also in relation to a deactivation process of the catalyst with time. Spectroscopic studies of the catalytic reaction indicate the formation of monomeric iridium-solvent adducts, which are likely to be the initiators of the polymerization reaction. A possible reaction mechanism is proposed, according to the data reported in the literature and the results of the present investigation. (C) 2002 Elsevier Science B.V. All rights reserved.