The initiation of ethylene polymerization was studied using Cr/SiO2 catalysts that differed in their Cr content. The interaction between ethylene and the catalyst surface was followed by FTIR transmission spectroscopy after each sequential injection of a small amount of ethylene into the IR cell. The results show that an ethylidene surface species, (CH3HC=Cr), initiates the polymerization. The polymer chain formed during the early reaction stage has a linear structure with a methyl group on one end and the Cr=CH- group on the other end. The number of active sites on the catalyst surface was estimated after each ethylene injection from the integrated absorbances of the nu(as)(CH2) band and the number of ethylene molecules added to the IR cell. The maximum concentration of active sites is observed when the total chromium content reaches 1 wt%. The results show that the population of active sites decreases during early reaction to a stable number identical for all catalysts with Cr content greater than or equal to 1 wt%.