In　this　work,　Co3O4/CeO2　was　synthesized　not　only　as　a　catalyst　precursor　but　also　as　a　model　catalyst　to　investigate　the　active　entities　over　the　water　gas　shift　reaction　(WGSR).　A　correlation　is　established　between　catalytic　activity　and　cobalt　entities　that　are　present　upon　the　surface.　The　existence　of　surface　Co2+　and　oxygen　vacancies　is　essential　for　efficient　WGSR.　It　is　observed　that,　at　200-300　°C,　there　is　the　transformation　of　Co3O4/CeO2　to　CoO/CeO2-x,　with　the　formation　of　extra　surface　oxygen　vacancies.　At　300-400　°C,　a　CoO1-x/CemCo1-mOn　phase　highly　active　for　the　WGS　reaction　is　formed.　According　to　DFT　atomistic　thermodynamics　calculation,　the　reconstruction　of　active　cobalt　species　at　equal　oxygen　chemical　potential　facilitates　the　adsorption　of　CO　and　H2O　at　specific　sites,　resulting　in　the　superior　catalytic　performance.　©　2019　American　Chemical　Society.