High temperature solution calorimetry using a (2 \mathrm{PbO} \cdot \mathrm{B}{2} \mathrm{O}{3}) solvent at (977 \mathrm{~K}) was applied to the high (T_{\mathrm{c}})-related compounds, (L n_{2} \mathrm{O}{3}(L n=\mathrm{Nd}-\mathrm{Lu}), T^{\prime}-L n{2} \mathrm{CuO}{4}(L n=\mathrm{Nd}-\mathrm{Gd})), and (L n{2} \mathrm{Cu}{2} \mathrm{O}{5}(L n=\mathrm{Dy}-\mathrm{Lu})). The heat of solution, (\Delta H_{5}), of (L n_{2} \mathrm{O}{3}) becomes less exothermic with decreasing size of (L n^{3+}) from (\sim-130) (\mathrm{kJ} /) mole in (\mathrm{La}{2} \mathrm{O}{3}) to (\sim-30 \mathrm{~kJ} /) mole in (\mathrm{Lu}{2} \mathrm{O}{3}). The heats of solution of rare earth oxides containing unevenly filled " (4 f) " orbitals are slightly more endothermic than those predicted by a straight line relating (\Delta H{\mathrm{s}}) to reciprocal ionic radius, (1 / r), for (\mathrm{La}, \mathrm{Gd}, \mathrm{Y}), and (\mathrm{Lu}), presumably reflecting the crystal field stabilization energy of (L n^{3+}) (CFSE) in the solid. The heat of solution of the (T^{\prime}) phase when plotted against (1 / r) shows a maximum value at (L n=\mathrm{Sm}). This may be explained by the CFSE and/or by a nonlinear change in lattice energy of the (T^{\prime}) phase as a function of (1 / r). The stability relations between (T^{\prime}) and (L_{2} \mathrm{Cu}{2} \mathrm{O}{5}) structures are discussed using the thermochemical data obtained. The data generally confirm patterns of stability seen in synthesis experiments. 1993 Academic Press. Inc.