CeO2-supported　Pt　single-atom　catalysts　have　been　extensively　studied　due　to　their　relevance　in　automobile　emission　control　and　for　the　fundamental　understanding　of　CeO2-based　catalysts.　Though　CeO2-supported　Pt　nanoparticles　are　often　more　active　than　their　single-atom　counterparts,　the　former　could　easily　redisperse　to　Pt　single　atom　under　oxidizing　diesel　conditions.　Therefore,　to　maximize　the　reactivity　of　every　Pt　atom,　it　is　important　to　fully　understand　the　reaction　mechanism　of　CeO2-supported　Pt　single　atoms.　Here,　we　report　a　CO　oxidation　study　on　a　Pt/CeO2　single-atom　catalyst,　where　we　can　account　for　all　of　the　neighbors　using　in　situ　and　operando　spectroscopy　techniques　and　microcalorimetric　measurements.　Coupled　with　density　functional　theory　calculations,　we　present　a　comprehensive　picture　of　the　dynamics　of　the　surface　species,　the　role　of　surface　intermediates,　and　explain　the　observed　reaction　kinetics.　We　started　with　a　catalyst　containing　exclusively　single　atoms　and　used　in　situ/operando　spectroscopy　to　provide　evidence　for　their　stability　during　the　reaction　and　to　identify　the　Pt1　complexes　before　and　during　the　reaction　and　their　binding　to　CO.　The　results　reveal　that　in　the　precatalyst,　Pt　is　present　as　Pt(O)4　on　the　CeO2(111)　step　edge　sites,　but　during　CO　oxidation,　we　find　that　two　Pt1　complexes　coexist,　representing　two　states　of　the　same　active　site　in　the　reaction　cycle.　The　dominant　state/complex　remains　Pt(O)4,　which　adsorbs　CO　very　weakly　as　shown　by　CO　microcalorimetry.　The　second,　minority　state/complex,　Pt(CO)(O)3　is　generated　through　the　reaction　of　Pt(O)4　with　CO,　and　CO　is　bound　strongly　to　Pt1.　Labile　oxygen　adatoms　from　the　CeO2　surface　play　a　major　role　in　the　regeneration　of　Pt(O)4　either　directly　from　Pt(O)3　or　by　reaction　with　the　strongly　adsorbed　CO　in　Pt(CO)(O)3.　We　show　that　the　formation　of　an　oxygen　vacancy　and　generation　of　a　labile　O∗　are　not　barrierless,　which　explains　the　long　lifetime　of　Pt(CO)(O)3　and　its　detectability　despite　being　a　minority　complex.　The　results　help　to　develop　a　comprehensive　view　of　the　dynamic　evolution　of　Pt1　complexes　along　the　reaction　cycle　and　provide　mechanistic　insights　to　guide　the　design　of　Pt-based　single-atom　catalysts.　©　2021　American　Chemical　Society.