The optical properties of color centers on the surface of MgO nanoparticles were studied by UV/vis reflectance spectroscopy. For color center formation, the MgO nanoparticles were exposed in the presence of hydrogen to polychromatic UV light. Essentially two absorption features at 1.8 and 2.4 eV were observed, with their relative intensities depending on the applied hydrogen pressure. during UV irradiation. This is explained by changes in the relative abundance of hydride groups stemming from H-2 chemisorption at different surface defects. The photolysis of irreversibly formed hydride groups results in the low-energy absorption at 1.8 eV, whereas the UV irradiation of a second type of hydride contributes to the appearance of a broad feature around 2.4 eV, which is composed of more than one band. Bleaching of color centers with N2O and subsequent addition of hydrogen generates atomic hydrogen as exclusive electron source for the MgO surface. In addition to the bands observed before, the appearance of a new band at 3.2 eV indicates that the underlying electron trap is not involved in hydrogen chemisorption reactions, but is capable of H atom oxidation. On the basis of previous infrared spectroscopic findings, a mechanism for the trapping process of hydrogen-derived electrons on the MgO surface is proposed.