The study of the real electronic spectrum of antiferromagnetic crystal of NiO is an actual problem of the solid state physics. Solid solution Ni x Mg 1- x O may be used as good model system to receive the additional information about optical transitions in NiO. Ni x Mg 1- x O solid solutions were studied in the present work using optical spectroscopy methods. For better understanding of properties it is significant to distinguish p - d and d - d charge transfer (CT) transitions. The photoluminescence (PL) measurements were made on the samples of solid solutions Ni x Mg 1- x O with rock salt crystal structure in the energy range of 3.7-12 eV. The edge of excitation spectrum of NiO nano very well coincides with PL excitation spectrum of Ni 0,008Mg 0,992O. For small Ni concentrations in Ni xMg 1- x O we consider only p-d CT transitions, whereas d - d CT transitions have small chance of existence. Thus observed coincidence between the edge of PL excitation spectrum of Ni 0.008Mg 0.992O and NiO provides reliable confirmation that the edge of PL excitation band in NiO arises due to p - d CT transitions.