Evaporation of Cs atoms onto dehydrated polycrystalline MgO leads to the formation of surface color centers in correspondence of surface point defects. EPR spectroscopy has revealed that the adsorbed Cs atoms are partially ionized, and a fraction of the electron spin density is delocalized onto a surface oxygen vacancy or trap. The observed defect can thus be written as Csdelta+(trap)(delta-). These results give evidence of the preferential interaction of the metal atoms with specific surface defect sites in the early stages of the metal support interaction. The reaction of these centers with molecular oxygen leads to bleaching of the surface with formation of the O-2(-) superoxide radical anion. A fraction of the adsorbed superoxide ions are adsorbed on "regular" Mg2+ sites while the remaining ones are adsorbed on top of Cs+ ions.