Various types of surface color centers at the surface of high surface area calcium oxide have been investigated by electron paramagnetic resonance. The defect centers, basically consisting in surface-trapped electrons, were generated either by UV irradiation of the solid in hydrogen (or deuterium) atmosphere or by addition to the oxide of low ionization energy metal (Mg,K) vapors. in the former case F-s(H)(+) defects have been obtained and characterized by magnetic interaction between the trapped electron and the proton of a nearby surface hydroxyl. In the second case F-s(+) centers and, in the case of the addition of potassium, a different center labeled F-s(K)(+) and characterized by interaction ofthe unpaired electron and a single potassium nucleus have been observed. The reactivity of all centers with oxygen to form adsorbed superoxide radical anions has been also investigated. The structural features ofthe adsorbed superoxide ion have been determined in the case ofthe reaction between F-s(H)(+) centers and O-2, again on the basis of the observed magnetic interaction between the adsorbed moiety and a nearby hydoxyl group. The reported data are strictly analogous to earlier data on MgO so that the whole set of experimental results can be generalized and interpreted in terms ofthe global surface defective properties ofthe alkaline-earth metal oxides.