Differential thermal analysis and composition, texture, and coverage measurements were carried out and gave evidences for chemical interaction between chromium oxide and the silica-alumina support of an olefin polymerization catalyst. Such interactions are related to the activity vs. propene polymerization and these relations allow to suggest that the active centers involve a layer of trivalent chromium oxide linked to the support by chromosiloxan bonds and covered with one or more layers of hexavalent chromium oxide.