Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) has been used to study the reduced Cr/SiO2 polymerisation catalyst during reaction with ethylene, isobutene, and carbon monoxide at ambient temperature and at 100 degrees C. When ethylene is added to the catalyst at ambient temperature, two kinds of coordinated ethylene are detected. Relatively stable ethylene-chromium complexes are formed prior to polymerisation which are stable during polymerisation, Also a more labile ethylene-chromium complex is detected by a transient band al 3004 cm(-1). The intensity of this band is reduced simultaneous to the growth ire the intensity of bands stemming from the polymer formed. Gel permeation chromatography (GPC) analysis of the polymer formed with an ethylene to chromium molar ratio of 1.0 shows that only 0.1% of the chromium is active at the conditions used. The low abundance of active sites suggests that the lack of observable end groups is due to the low number of end groups present, It also means that mechanistic investigation of this catalyst system by spectroscopic methods is difficult and that misinterpretations may easily be done, Saturation of the catalyst with a non-polymerising olefin such as isobutene before introducing ethylene shows that isobutene coordinates in a similar manner as ethylene does, However, coordinated isobutene does not block ethylene polymerisation, and a similar transient ethylene species is observed for the catalysts saturated with isobutene.