The photoluminescence, its excitation and absorption spectra of MgO single crystals are investigated before and after the irradiation with fast neutrons. It is shown that the fast neutron irradiation leads to the formation of two types of centers: the interstitial proton H + i and the Mg(OH) 2 microphase. From the behaviour of the photoluminescence (PL) and its excitation (PLE) spectra we suppose that the photoluminescence band at 730 nm of the MgO crystals belongs to the complex V − OH − Fe 3+ centers, which is transformed during the irradiation with fast neutrons. The obtained results allow assume that the transitionions play an important role in the migration of vacancies and interstitials.