Gas-phase clusters isolated in vacuum have been studied intensively as a model system of heterogeneous catalysts. Recently, reactivity of clusters in thermal conditions attracts much attention, as catalysts operate in thermal conditions. In this review, a new experimental method to observe reactions of thermalized clusters, gas-phase temperature-programmed desorption (TPD), is introduced. As a typical example, redox reactions of cerium oxide clusters have been revealed using this method. Oxygen storage capacity of cerium oxide clusters was confirmed, i.e., cerium oxide clusters can release and extract oxygen. Oxidation reactions of CO and NO by cerium oxide clusters were observed, and detailed processes of these reactions were analyzed by gas-phase TPD.