Copper　supported　on　ceria　prepared　by　the　impregnation　method　was　used　as　a　model　catalyst　to　investigate　the　synergetic　mechanism　between　copper　species　and　ceria　for　NO　reduction　by　CO.　To　identify　the　copper　species　in　the　catalyst,　we　treated　the　as-synthesized　Cu/CeO2　by　30　wt.%　nitric　acid　solution.　Fresh　and　nitric　acid　treated　Cu/CeO2　catalysts　were　characterized　and　compared　with　AAS,　XRD,　TPR,　EPR,　Raman　and　NO-TPD　techniques.　It　is　found　that,　after　nitric　acid　treatment,　there　is　only　0.27　wt.%　Cu　left　(vs.　5　wt.%　Cu　in　fresh　sample);　this　is　present　in　the　following　forms:　(1)　isolated　copper　ions　in　octahedral　sites　of　ceria,　(2)　copper　oxide　clusters　and　(3)　copper　ions　in　ceria　lattice.　The　residual　small　amount　of　Cu　has　a　significant　effect　on　surface　structure,　redox　property　and　catalytic　behavior　of　ceria,　indicating　that　these　copper　species　are　crucial　copper　species　inducing　copper-ceria　interactions.　Moreover,　it　is　suggested　that　the　relative　free　movement　of　oxygen　from　ceria　to　supported　copper,　caused　by　the　three　copper　species,　leads　to　oxygen　vacancies　in　ceria,　significantly　enhancing　the　NO　conversion　of　Cu/CeO2　during　NO　+　CO　reaction.　(C)　2010　Elsevier　B.V.　All　rights　reserved.