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 degrees C, there is the transformation of Co3O4/CeO2 to CoO/CeO2-x, with the formation of extra surface oxygen vacancies. At 300-400 degrees 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.