A theory has been developed to describe the diffusion of trivalent impurities in a divalent metal oxide and has been evaluated for the diffusion of trivalent chromium in NiO. The theoretical model developed by Lidiard to describe the diffusion of divalent impurities in alkali halides was revised to describe the migration of chromium-vacancy complexes in the divalent NiO crystal. A radioactive⁵¹Cr tracer and NiO single crystals were used and the concentration profiles were analyzed by sectioning and counting. The diffusion coefficient of chromium as a function of chromium concentration was determined at 1000°, 1100°, 1200°, and 1300°C and at oxygen pressures of 1.0, 10^-2, and 10^-5 atm using a modified Matano analysis of the concentration profiles. A reasonable fit of the theory to the experimental concentration dependence of chromium diffusion was obtained which indicates that, at 1000°C and above, chromium diffuses in NiO by a mechanism involving chromiumvacancy two-member complexes. The energy of association of a chromium-vacancy couple was found to increase from approximately -3000 cal at 1000°C to about -600 cal at 1300°C. The diffusivity of chromium (Dcr) in NiO saturated with chromium increases from approximately 2 x 10^-12 cm²/s at 1000°C to 2 x 10^-10 cm²/s at 1300°C.