Chromium catalysts, prepared with different chromium contents, supports and precursor compounds, were characterized by O-2 and CO chemisorption, in order to quantify the Cr3+ and Cr2+ sites, active species to dehydrogenation and ethylene polymerization reactions. Both probe molecules, O-2, and CO, showed to be selective to the chromium active species, Cr3+ and Cr2+, respectively, and a valid method to quantify these sites. Distinct chromium species, Cr6+, amorphous Cr3+, crystalline Cr3+ and Cr2+, were characterized by XRD, DRS, TPR, FT-IR and DRIFTS in the calcined and reduced catalysts. The distribution of chromium species, which depends on the content, precursor compound and support, was related to the catalytic activity in the dehydrogenation and polymerization reactions. The catalytic activity to cyclohexane dehydrogenation depends on the dispersion of amorphous Cr3+ species. The activity in the ethylene polymerization is related to the amount of Cr-A(2+) and Cr-B(2+) species. Both sites showed similar activities. (C) 2003 Elsevier Science B.V. All rights reserved.