An active surface science model for the Phillips ethylene polymerization catalyst has been prepared by impregnating aqueous CrO3 on a flat silicium(100) substrate covered by amorphous silica. Using a combination of X-ray photoelectron spectroscopy, secondary ion mass spectrometry, and Rutherford backscattering spectrometry, we studied the effect of calcination on the state of the supported chromium. Depending on the calcination temperature and the initial Cr loading;of the catalyst, two processes are observed. The impregnated chromate anchors to the silica surface in an esterification reaction with the surface silanol groups of the support. The saturation coverage of these surface chromates is 2.4 Cr/nm(2) for a calcination temperature of 450 degreesC. Superficial, not anchored, chromate slowly desorbs from the flat silica surface. Under crowded conditions a portion oi the surface chromates also desorb if the calcination temperature is increased, while low Cr loadings (>1 Cr/nm(2)) are stable up to the highest calcination temperature in our experiments (730 degreesC). The silica-bound surface chromates are monochromates exclusivly, independent of the initial loading or calcination temperature.