The stability of LnMO 3 against the disproportionation to garnet plus sesquioxide is controlled almost entirely by H and P V but not by T S. On the contrary, the stability of Ln 3 M 5 O 12 against disproportionation to perovskite plus sesquioxide is controlled not only by H and P V but also by T S. The P-T boundary between Ln 3 M 5 O 12 and 3LnMO 3 Ψ M 2 O 3 has a negative slope. The positive S and negative V for the disproportionation are caused by an increase in coordination number and an increase in bond distance. H of perovskite formation is mainly controlled by two factors, the strengthening of the ionic bond in Ln 2 O 3 with decreasing ionic radius of Ln 3Ψ and the weakening of the ionic bond between Ln and the distant four O atoms in LnMO 3 with decreasing ionic radius of Ln 3Ψ . H of garnet formation is mainly controlled by two factors, the strengthening of the ionic bond in Ln 2 O 3 with decreasing ionic radius of Ln 3Ψ and the deviation of the ionic radius of Ln 3Ψ from the optimum size for the garnet structure. S values of both perovskite formation and garnet formation are deduced to be negative, which suggests that Ln 2 O 3 phases possess relatively large entropies.