Preliminary results on the activity of metallocene supported over a polyhedral oligomeric silsesquioxane (POSS) modified silica for ethylene polymerization (Bianchini et al J Polym Sci Part A Polym Chem 2005 43 5465-5476) were extended showing the effect of large variations of the amount of both POSS and metallocene on the catalyst activity Diffuse reflectance infrared spectroscopy results showed that at low concentrations (<1 wt %) POSS was physically adsorbed on SiOH groups present in the silica surface through H bonds At larger concentrations POSS was adsorbed by covalent bonds with the silica surface The metallocene catalyst was adsorbed either on the silica surface by interactions with residual SiOH groups or in the organic branches of the POSS It was found that the highest contribution to the catalyst activity arose from the metallocene directly adsorbed on the silica support The effect of POSS to increase the catalyst activity was mainly due to its effect as a horizontal spacer of metallocene molecules adsorbed on the silica surface An optimum range of POSS and metallocene adsorbed on the silica support was found that gave catalyst activities close to the one of the homogeneous system while doubling the molar mass of the polyethylene pro duced (C) 2010 Wiley Periodicals Inc J Polym Sci Part A Polym Chem 48 5938-5944 2010