Gas-phase cluster reactivity experiments, combined with theoretical calculations, provide insight into how factors such as size, stoichiometry and charging effects influence reactions promoted by metal and metal oxide catalysts. In this Letter, we describe the use of clusters to investigate the role of different types of oxygen centers in catalytic oxidation reactions. First, we focus on recent results from our laboratory concerning the oxidation of carbon monoxide by gold clusters and how the charge state of the cluster determines the mechanism of oxidation. We then discuss how radical oxygen centers in zirconium oxide clusters promote the oxidation of carbon monoxide, ethylene and acetylene and how charge state may be used to tune the selectivity of these reactions. An overview of future directions for the research is given in conclusion. (c) 2009 Published by Elsevier B. V.