A series of CrOx/SiO2/Si(100) model catalysts were tested for ethylene polymerization activity, varying chromium loading, and calcination temperature. Chromium coverage of the model catalyst, quantified by Rutherford backscattering spectrometry, decreases with increasing calcination temperature as some chromium desorbs from the silica support. The polymerization activity of the model catalysts is correlated to calcination temperature and chromium coverage. Based on the evidence presented, we propose that high local Cr coverage-short Cr-Cr distances-is detrimental to polymerization activity, possibly because it facilitates dimerization leading to inactive chromium sites. Calcination at high temperatures not only causes depletion of surface silanol groups, but may also facilitate the formation of isolated chromium sites, which can evolve into active polymerization centers. AFM images of nascent polymer films after short polymerization times offer a means to visualize the distribution of polymerization activity on the silica surface. They indicate that the catalytically active chromium forms islands on the silica surface. (C) 2004 Elsevier Inc. All rights reserved.