One crucial requirement for the operation of proton exchange membrane fuel cells (PEMFC) is to feed carbon monoxide free hydrogen to the anode. This need can be achieved by using catalysts able to selectively oxidize CO in the presence of excess hydrogen. Herein we report the preferential CO oxidation (PROX) in the presence of hydrogen over Pt/Ce_xZr_x−1O₂ (x=0, 0.15, 0.5, 0.68, 1) catalysts. A comparison with results observed on a Pt/Al₂O₃ catalyst is also presented. We examined the effect of temperature (90–300 °C) and O₂ excess (λ=0.8–2). Ceria-supported platinum catalysts were more active than Pt/Al₂O₃ in both CO and H₂ oxidation. The result was a sharp “light off” around 90 °C for the oxygen conversion. The maxima, which appeared in the CO conversion and in the selectivity toward CO oxidation as a function of temperature on Pt/Al₂O₃, did not show up in the case of ceria-supported samples. Chloride ion-containing Pt/CeO₂ catalysts showed lower performances in the PROX reaction, especially at low temperatures. Four types of reaction mechanisms were suggested for Pt/Ce_xZr_x−1O₂ samples: (i) competitive Langmuir–Hinshelwood CO and hydrogen oxidation on Pt particles, (ii) noncompetitive Langmuir–Hinshelwood mechanism on the metal/oxide interface, (iii) hydrogen oxidation on the support, and (iv) water–gas-shift reaction at high temperatures. The second reaction route predominated at low temperatures (90–130 °C) and was found preferential to CO oxidation rather than hydrogen–oxygen reaction. This process was selectively blocked by Cl ions. The possible application of Pt/CeO₂ and Pt/Al₂O₃ catalysts was discussed. Ceria appeared as a suitable support for the preferential CO oxidation catalysts at low temperatures.