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 (Pt_NP-S) and 36 nm (Pt_NP-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; Pt_NP-S in Pt_NP-S/TiO₂ appear nearly oxidized state while about half of Pt is metallic state in Pt_NP-S/Al₂O₃; Pt_NP-L in both Pt_NP-L/TiO₂ and Pt_NP-L/Al₂O₃ exhibit metallic state. All the Pt species supported on TiO₂ present much lower apparent activation barriers (E_app) than that on Al₂O₃. Moreover, Pt₁/TiO₂ possesses dozen times of efficiency than Pt_NP-S/TiO₂ although they have similar E_app 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.