A 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. Impregnating in a spin-coating technique offers control over the chromium loading, which was varied between 0.4 and 4.0 Cr/nm2. After impregnation chromium is dispersed molecularly at loadings smaller than 2 Cr/nm2. Upon calcination chromium is anchored to the support over Cr - O - Si ester bonds, although a fraction is lost from the surface due to volatilization, as could be established with X-ray photoelectron spectroscopy. The model catalyst polymerizes ethylene at 160 °C and atmospheric pressure after being activated in 80/20 He/O2 at 650 °C, confirming its validity as a model for the industrial catalyst.