AbstractIn polymerizations with ethylene and copolymerizations with ethylene and 1‐butene the catalyst systemrac‐ethylene(bisindenyl)zirconium dichloride (rac‐Et(Ind)2ZrCl2)/methylaluminoxane (MAO) shows lower activities than the analogous Hafnium catalyst. The polymers obtained with the hafnium‐catalyst have average molar masses up to ten times higher than those synthesized with the zirconium compound. It is remarkable thatrac‐Et(Ind)2HfCl2is much more able to incorporate the comonomer. This is expressed by the copolymerization parameters. At a polymerization temperature of +30°C, e.g., ther1‐parameter of the hafnium‐catalyst is 5,4, the corresponding parameter of the zirconium catalyst is 19,4. Polymerizations with a catalyst mixturerac‐Et(Ind)2(Hf, Zr)Cl2(containing 95%rac‐Et(Ind)2HfCl2and 5%rac‐Et(Ind)2ZrCl2) show that each catalyst compound produces its own p