Cr/SiO2 catalysts for ethylene polymerization, prepared with 1 wt.% Cr and different chromium precursors, were characterized by temperature-programmed reduction (TPR) and desorption of carbon monoxide. Complete reduction of Cr6+ to Cr2+ species was observed with CO at 623 K, but the reduction profile modified in the presence of Cr3+, depending on the chromium precursor. The desorption of CO occurred in relatively low temperature for catalysts without Cr 3+' independent of the Cr-A(2+) /Cr-B(2+) active sites ratio. The desorption of CO was observed at higher temperature in the catalysts containing Cr3+ suggesting modification of the Cr2+-CO adsorption strength in the presence of Cr3+. The desorption of CO2 was detected in all catalysts, associated to the CO disproportionation reaction at lower temperatures or to the decomposition of carbonates and carboxilates species above 500 K. The desorption of CO agreed with polymerization activity results, showing the importance of this technique to characterize active chromium sites. (C) 2004 Elsevier B.V. All rights reserved.