The aim of this work is to study the influence of the addition of different oxides to an alumina support, on surface acidity and platinum reducibility in platinum-based catalysts, as well as their effect on the activity and selectivity in CO preferential oxidation, in presence of hydrogen. A correlation between surface acidity and acid strength of surface sites and metal reducibility was obtained, being Pt-support interaction a function of the acid sites concentration under a particular temperature range. In platinum supported on alumina catalysts, CO oxidation follows a Langmuir-Hinshelwood mechanism, where O-2 and CO compete in the adsorption on the same type of active sites. It is noteworthy that the addition of La2O3 modifies the reaction mechanism. In this case, CO is not only adsorbed on the Pt active sites but also on La2O3, forming bridge bonded carbonates which leads to high reactivity at low temperatures. An increase on temperature produces CO desorption from Pt surface sites and favours oxygen adsorption producing CO2. CO oxidation with surface hydroxyl groups was activated producing simultaneously CO2 and H-2. (C) 2009 Elsevier B.V. All rights reserved.