Oxidative dehydrogenation of propane and isobutane has been investigated over CrOx/silica catalysts prepared by wet impregnation (Cr loading: from 0.5 to 7.5 wt%) using carbon dioxide as oxidant. In particular, reaction conditions were set to attain high conversions, similar to that required in industrial processes. The highest activity, selectivity and stability were exhibited by a catalyst with a Cr loading of 5.0 wt%. DR UV-vis studies indicated that Cr(VI) ions in the form of mono-, di- and polychromates were present in all catalysts, while Cr(III) oxide (alpha-Cr2O3) Was found only for Cr loading higher than 3.0 wt%. The catalyst activity seemed correlated to more dispersed chromates. Introduction of oxygen in the reaction mixture up to 5.0 vol.% resulted in a significant increase of catalysts activity and stability. Some decrease in the propene and isobutene selectivity occurred, but because of the increase of the selectivity towards lighter olefins, the total olefin selectivity increased. (C) 2008 Elsevier B.V. All rights reserved.