CO oxidation on Pt/CeXZr1-XO2 (X = 0, 0.15, 0.5, 0.68, 0.8 and 1) was studied, both in the absence of H-2 and in H-2-rich streams. Catalyst activity was correlated with support reducibility and Pt content. A certain excess of oxygen (lambda > 1) with respect to CO had to be fed to the system in order to obtain complete CO conversion in the presence of H2. Three catalysts were found capable of complete CO depletion with effective oxygen use at lambda = 2: Pt/Ce0.8Zr0.2O2, Pt/Ce0.68Zr0.32O2 and Pt/Ce0.5Zr0.5O2. More demanding working conditions, by increasing GHSV from 12,000 up to 18,000 h(-1), reduced the adequate catalysts to just two: Pt/Ce0.68Zr0.32O2 and Pt/Ce0.5Zr0.5O2, and only the latter would be adequate if the conditions were still more demanding. The presence of CO2 and H2O, produced a general decrease of CO conversion for the catalysts supported on mixed oxides and, among them, only Pt/Ce0.5Zr0.5O2 remained capable of complete CO removal with effective oxygen use at lambda = 2. However, the presence of CO2 and H2O increases CO conversion of Pt/CeO2 such that this catalyst arises as an alternative. Choice between Pt/CeO2 and Pt/Ce0.5Zr0.5O2 requires further research, and may depend on the actual application. (C) 2006 Elsevier B.V. All rights reserved.