CO and C3H6 oxidation have been carried out in the absence and presence of water over a Pd/Al2O3 catalyst. It is clear that water promotes CO and, as a consequence, C3H6 oxidation takes place at much lower temperatures compared with the dry feed. The significant increase in the catalyst's activity with respect to CO oxidation is not simply associated with changes in surface concentration as a result of competitive adsorption effects. Utilising O-18(2) as the reactant allows the pathways whereby the oxidation due to gaseous dioxygen and where the water activates the CO and C3H6 to be distinguished. In the presence of water, the predominant pathway is via water activation with (CO2)-O-16 and (COO)-O-16-O-18 being the major species formed and oxidation with dioxygen plays a secondary role. The importance of water activation is further supported by the significant decrease in its effect when using D2O versus H2O. (C) 2013 The Authors. Published by Elsevier B.V. All rights reserved.