The mixing properties of solid solutions of post-spinel phases of the composition CaCr2O4–CaAl2O4, CaCr2O4–CaFe2O4, MgCr2O4–MgAl2O4 and MgCr2O4–MgFe2O4 in the temperature range 1873– 2223 K and pressures 18–25 GPa were studied using the method of semi-empirical modeling. With these PT parameters, the energies of formation of impurity defects of trivalent metal ions (aluminum and iron) in isomorphic sites were estimated. It is shown that (1) the studied binary solid solutions are characterized by complete miscibility, (2) the incorporation of the Fe3+ impurity ion into the post-spinel phases of MgCr2O4 and CaCr2O4 is less favorable in terms of energy than the incorporation of Al3+ ions over the entire range of pressures and temperatures under study. The results obtained were used to interpret the composition of post-spinel phases forming inclusions in lower mantle diamonds.