Fuel cells have risen as a clean technology for power generation and much effort has been done for converting renewable feedstock in hydrogen. The water-gas shift reaction (WGS) can be applied aiming at reducing the CO concentration in the reformate. As Pt/CeO2 catalysts have been pointed out as an alternative to the industrial WGS catalysts, the modification of such systems with magnesium was investigated in this work. It was shown that the addition of MgO to Pt/CeO2 increased the activity and stability of the catalyst irrespective of the preparation method used, either impregnation or co-precipitation. Based on TPR and IR spectroscopy experiments, it was seen that the presence of magnesium improved ceria reduction favoring the creation of OH groups, which are considered the active sites for the WGS reaction. The evolution of the surface species formed under reaction conditions (CC, H2O, H-2) observed by DRlF-rS evidenced that the formation of formate species and the generation Of CO2 is closely attached to each other; under a reaction stream containing hydrogen the presence of formate species showed to be more relevant while the CO2 formation was hindered. It is suggested that the addition of MgO favors the formate, decomposition and lower the carbonate concentration on the catalyst surface during WGS reaction. (c) 2007 Elsevier B.V. All rights reserved.