AbstractA study of the nature of polymerization catalysis over CrO3–silica and of the nature of the polyethylene obtained is presented. A fixed surface chromate (or possibly dichromate) species was shown to be activated for polymerization by an oxidation–reduction reaction with the monomer or with other reactive compounds such as CO. A change in catalyst color from orange to blue occurred simultaneously, and an indigoblue color was present only during ethylene addition, indicating involvement of Cr d orbitals. A spectacular chemiluminescence, due to excitation of oxygen, occurred when CO‐treated catalyst was exposed to air. Active site population and the rate at which each site produced polymer molecules were calculated. A reaction mechanism compatible with the experimental data is depicted.