Investigation of the polymerization of ethylene on CO-reduced Phillips catalyst (1 wt% chromium) by infrared spectroscopy reveals the presence of new OH bands. In particular, an OH-band appears at 3,605 cm(-1), consistent with the interaction of the SiOH group with an adjacent Lewis acidic chromium center, Si-(mu-OH)-Cr. Polymerization with d(4)-ethylene leads to the formation of the isotopically shifted band at 2,580 cm(-1), consistent with heterolytic C-H activation of ethylene over a Cr-O bond to generate the first Cr-C bond in ethylene polymerization with Phillips catalyst, as recently observed on well-defined Cr(III) silicates.