Abstract CO oxidation over Pd/WO 3 films prepared on a glass substrate has been examined at the substrate temperature of 150 – 250 °C and pressures less than 1 Torr with a stoichiometric mixture of CO and O 2 . Under the given reaction condition, the chemical states of the Pd/WO 3 film gradually change into the most catalytically active form with the highest saturation reaction rate regardless of the initial oxidation states. The measured CO oxidation rate over the Pd/WO 3 is strongly dependent on the chemical states of Pd and W. Either metallic Pd or fully oxidized PdO phase is not as catalytically active as the active form with mixed metallic Pd and thin PdO layers supported on WO 3 with partially reduced W 5+ state which is spontaneously obtained during the catalytic reaction cycles. Our results indicate that the facile oxygen transfer between Pd and WO 3 layers not only facilitate the spontaneous changes into the active form, but also act as a promotional role in CO oxidation over the Pd layer.