In the present work, interdiffusion between NiO and CoO-doped MgO has been investigated by the solid state galvanic cell method in the temperature range 1273-1439 K. The compositional as well as temperature effects of doping on the rate of interdissolution were examined during the measurements. Evaluation of the interdiffusivity data from the cell EMF values required a knowledge of the activity of NiO in the ternary oxide system CoO-MgO-NiO. An assessment of the thermodynamics of this system was made from the literature data on the relevant binary systems using a two sublattice model. From the EMF values and the thermodynamic information, the interdiffusivities were calculated using a theoretical model developed in the division earlier. The results indicate that, at low concentrations of the dopant, the changes in the interdiffusivities are not significant. On the other hand, at 10 mol% CoO in the starting CoO-MgO solid solution in contact with 50 mol% NiO, an increase in the rate of diffusion was noticed.