Mesoporous molecular sieves, with pore diameters of 2.6-25 nm, were impregnated with methylaluminoxane and bis(butylcyclopentadienyl)zirconium dichloride and tested as catalysts for the gas-phase homopolymerization of ethylene at ethylene pressures of 200 psi and temperatures of 50-100 degreesC and for 1-hexene/ethylene copolymerization at 70 degreesC. The activities and activity profiles, at constant Zr and Al contents, depended on the pore size of the supports and the polymerization temperature. Maximum activities for both the homopolymerizations and copolymerizations were observed for catalysts made with supports having pore diameters of 2.6 and 5.8 nm. Homopolymerization activities were highest at temperatures of 70-80 degreesC; average homopolymerization and copolymerization activities up to 9000 kg of polyethylene/ (mol of Zr h) were obtained. The weight-average molecular weights (Momega's) were not a function of the support pore size but decreased with increasing reaction temperatures, from about 260,000 at 50 degreesC to about 165,000 at 100 degreesC. The polydispersities were essentially constant at 2.5 +/- 0.2 for the homopolymers. Momega's for the 1-hexene/ethylene copolymers had an average value of 117,000 with an average polydispersity of 2.8. The amount of triisobutyl aluminum added to the reactor significantly affected the activity and activity profiles.