The polymerization properties of a well-defined flat CrOx/SiO2/Si(100) catalyst were investigated. To compare this model system with the conventional Phillips catalyst based on porous silica, polymerization reactions were carried out at various temperatures. In contrast to the conventional Phillips catalyst, our model showed constant activity from the start of polymerization. Both the activity and molecular weight were tenfold higher for the flat catalyst. As expected, an increase in polymerization temperature led to an increase in activity; however, the molecular weight distribution was only mildly affected. At a temperature around the melting point of polyethylene, a sudden drop in both activity and molecular weight was observed. Based on our observations, we propose that chromium sites produce polymer chains in short bursts of high activity, followed by a dormant period, and that ethylene polymerization of active chromium sites can result in local ethylene depletions that limit both activity and molecular weight. This can severely affect the polymerization properties in the event of high active site density and reduced mass transfer through the formed polymer layer.