The dynamics of photoinduced formation of electron and hole centers at the surface of MgO powder has been investigated in situ using EPR spectroscopy. Monochromatic excitation of the sample with 282 nm photons leads to creation of well-separated electron and hole centers at the surface. Polychromatic (200-900 nm) irradiation leads to a very different time evolution of the EPR signal of the photoinduced centers. We argue that this difference is due to secondary reactions stimulated by the low energy part of the radiation spectrum. It can be explained by the excitation of the localized holes to the valence band and localized electrons into the conduction band and their consequent trapping at defect states and powder particle interfaces. The results demonstrate the existence of various types of EPR active and inactive electron and hole traps at the surface of MgO powder and provide preliminary data on their energy distribution.