Pt/CeO2　single-atom　catalysts　have　recently　attracted　increasing　interest　due　to　excellent　thermal　stability,　high　atom　efficiency,　and　high　activity　in　catalysis.　In　this　study,　by　means　of　density　functional　theory　(DFT)　calculations,　we　systematically　compare　the　stability　and　CO　oxidation　reactivity　of　Pt　single　atoms　supported　on　CeO2(111)　(Pt/CeO2)　and　Ga-doped　CeO2(111)　(Pt/Ga-CeO2).　It　was　found　that　the　formation　of　an　oxygen　vacancy　(OV)　is　very　facile　near　a　surface　Ga-doping　site　(Pt/Ga-CeO2-OV).　Significantly,　the　stability　of　Pt　single　atoms　anchored　on　the　Ga　site　was　enhanced　compared　with　those　on　the　bare　ceria　surface.　In　addition,　our　DFT　results　suggest　a　CO　oxidation　mechanism　on　Pt/Ga-CeO2-OV　that　differs　from　that　on　Pt/CeO2.　In　particular,　the　OV　site　plays　an　important　role　in　activating　the　oxygen　molecule,　which　then　reacts　with　CO　preadsorbed　on　Pt.　The　calculated　energy　barrier　on　Pt/Ga-CeO2-OV　is　about　0.43　eV　lower　than　that　on　the　undoped　catalyst,　suggesting　an　enhanced　reactivity　for　CO　oxidation.　Experiments　on　CO　oxidation　and　in　situ　diffuse　reflectance　infrared　Fourier　transform　spectroscopy　are　performed　to　corroborate　the　results　obtained　from　the　DFT　calculations,　and　a　good　agreement　is　achieved.　The　combination　between　calculations　and　experiments　sheds　light　on　the　influence　of　support　doping　on　atomically　dispersed　Pt/CeO2　catalysts.　©　2018　American　Chemical　Society.