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The source of activity of metal/metal oxides has always been an interesting,important but highly challenging research topic in heterogeneous catalysis.In CO oxidation reaction,this work clarifies dispersion and support dictated activities of Pt including single-atom (Pt1),2.8 nm (PtNP-S) and 36 nm (PtNP-L) Pt supported on both reducible TiO2 and “inert” Al2O3 supports.The X-ray absorption fine structure (XAFS) shows that chemical state of Pt is affected by both dispersion and support:Pt1 presents fully oxidized state in both Pt1/TiO2 and Pt1/Al2O3;PtNP-S in PtNP-S/TiO2 appear nearly oxidized state while about half of Pt is metallic state in PtNP-s/Al2O3;PtNP-L in both PtNP-L/TiO2 and PtNP-L/Al2O3 exhibit metallic state.All the Pt species supported on TiO2 present much lower apparent activation barriers (Eapp) than that on Al2O3.Moreover,Pt1/TiO2 possesses dozen times of efficiency than PtNP.S/TiO2 although they have similar Eapp values.A truth is finally made clear that a reducible metal oxide with low oxygen vacancy formation energy is critical to endow Pt/metal oxide a high activity and the single-atom dispersion of Pt is the way to maximize the active sites of Pt/metal oxide.