Identification　of　the　chemical　states　of　catalytic　sites　is　critical　for　an　atomic-level　understanding　of　catalytic　mechanisms.　Herein,　hydrogen　thermal　pretreatment　of　the　Pt　single　atoms　on　porous　nanorods　of　CeO2　(Pt-1/PN-CeO2)　induced　the　formation　of　isolated　bimetallic　PtCe　sites　as　a　new　type　of　active　center　for　CO　oxidation.　The　evolutions　of　Pt-1/PN-CeO2　catalysts　during　the　hydrogen　pretreatment　and　CO　oxidation　were　examined　by　various　in　situ　techniques　including　infrared,　ambient-pressure　X-ray　photoelectron　and　X-ray　absorption　spectroscopy.　The　experimental　results　demonstrate　that　these　bimetallic　sites　can　be　partially　preserved　or　reoxidized　into　Pt-O-Ce　with　a　low　coordination　number　with　oxygen　under　realistic　conditions,　leading　to　the　appropriate　CO　adsorption　and　activating　the　efficient　activity　of　Pt-1/PN-CeO2　for　CO　oxidation　at　low　temperature.