Methylalumoxane (MAO) is an active pre-catalyst for the polymerization of isobutylene and the copolymerization of isobutylene with isoprene at ambient temperature. The absence of any proton-containing substances suggests that the solvent dichloromethane reacts with the MAO forming the active cationic species. In the presence of toluene, as the solvent, the reaction rate is extremely low. The increase of the MAO concentration in the reaction mixture causes a large increase in the isobutylene conversion, although the molecular weight of the polymers sharply decreases. The use of CD2Cl2 as a solvent induces a large kinetic isotope effect (KIE=7). In the copolymerization of isobutylene with large amounts of isoprene a cross-linkage copolymer is formed. The polymerization and copolymerization reactions were also studied in the presence of a mixture of MAO with the complex (acac)2TiCl 2 (acac=acetylacetonate).