Bare and ceria-promoted MnO catalysts (0 <= ?<=1) were prepared by redox-precipitation reactions of Mn(VII), Mn(II) and Ce(III) or Ce(IV) precursors in slightly acidic (pH, 4.5) or basic (pH, 8.0) environment to assess genesis, nature, and functionality of surface active sites. Both synthesis protocols yield nanostructured materials with large surface area and exposure of Mn sites, featuring high activity in the CO oxidation and the phenol wet-air-oxidation (CWAO) model reactions (T, 423 K). High oxide dispersion prompts an extensive incorporation of Mn(II) ions into ceria substitutional solid-solution structures, forming oxygen-vacancies with stronger oxidation activity than surface Mn(IV) sites. Basic structure-activity relationships indicate that the superior CO oxidation performance of the pristine ?-MnO system relies on large exposure of very reducible Mn(IV) active sites, while O-adsorption onto Mn(II)/O-vacancy active centres generates very reactive surface oxygen-species boosting the efficiency of composite MnCeO catalysts in the CWAO of phenol.