A diffusion model is proposed for studying interdiffusion in binary oxide mixtures. The model has been applied to data obtained by dynamic galvanic cell measurements on NiO-MgO and CoO-MgO systems. Cationic interdiffusivities derived from EMF data are in reasonable agreement with those reported in the literature using the conventional diffusion couple technique. The dynamic nature of the present method offers scope for continuous monitoring of solid state reactions and related processes.