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The tie lines delineating ion-exchange equilibria between MCr2O4-MAI2O4 spinel solid solution, where M is either Mn or Co, and CrzO3-AI2O3 solid solution with the corundum structure were determined at 1373 K by electron microprobe and EDAX point count analysis of the oxide phases equilibrated with metallic Co and Au-5% Mn. The component activities in the spinel solid solutions are derived from the tie lines and the thermodynamic data for Cr,O3-AlZO3 solid solutions available in the literature. The Gibbs free energies of mixing calculated from the experimental data are discussed in relation to the values derived from the cation distribution model based on the site preference energies and assuming random mixing on both tetrahedral and octahedral sites. Positive deviations from ideality observed in this study suggest a miscibility gap for both series of spinel solid solutions at low temperatures in the absence of oxidation.