The promotional effect of Nb to Pt/Al2O3 Supported catalysts during the preferential oxidation of Co (PROX) was studied using various spectroscopic techniques. Addition of small amounts of Nb (<5%) stabilizes 40% of the loaded platinum as Pt2+, which remains oxidized even after reduction treatments. This Nb-promoted catalyst is very active and selective for the PROX reaction. On Pt/Nb2O5 and at high Nb loading for the Pt/Nb/Al2O3 catalysts, the selectivity to CO2 decreases and the selectivity for H-2 oxidation increases opposite to the selectivity observed at low Nb loadings. The increase CO2 selectivity due to Nb promotion is ascribed to the inhibition of CO at low temperature which decreases hydrogen oxidation. Operando FTIR results indicate the presence of adsorbed CO as well as carbonates, bicarbonates and formates during the PROX reaction. An IR band at 968 cm(-1) indicates the presence of Nb=O moieties at low Nb loadings. At higher Nb loadings, IR suggests the formation of three-dimensional Nb2O5 aggregates. The surface of the Nb containing catalysts is complex containing reduced and oxidized Pt which is modified by NbOx species either surrounding the Pt crystallites or decorating them. (C) 2008 Published by Elsevier Inc.