The interaction of O-2 and CO gas-phase molecules with oxygen vacancies on the MgO (100) surface has been studied by means of cluster models and ab initio wave functions. It is found that the surface oxygen vacancies, or F-s centers, exhibit a high reactivity toward O-2 and CO at variance with the regular MgO surface. The reaction proceeds through the formation of radical anions, O-2(-) and CO-, via the transfer of one electron trapped in the surface cavity to the empty levels of the by electrostatic forces. Although the mechanism of the interaction is the same for the two molecules, the details of the energetics are different. O-2 spontaneously removes the electrons trapped in the MgO oxygen vacancies to form the stable O-2(-) superoxide anion. On the contrary, CO- forms only at finite temperatures and is a metastable species. The different behavior can be rationalized in terms of electron affinities of the two molecules. The calculations are useful also for the spectroscopic characterization of the radical anions at the surface. The calculations of electron paramagnetic resonance (EPR) hyperfine coupling constants and, for CO, of the vibrational frequencies indicate that the experimental spectra are consistent with the existence of O-2(-) and CO- surface species. The analysis of the vibrational shifts shows that the coordination mode of CO is C-down and not O-down.