Solution calorimetry, using 6.0 M HCl as a solvent, is used to study the thermochemistry of Gd2BaCuO5 and the high-temperature superconductor LuBa2Cu3O6.92. For the first time, the standard formation enthalpies of these phases have been determined as follows: Delta H-f(o) (Gd2BaCuO5, s, 298.15 K) = -2618.6 +/- 7.4 kJ/mol; Delta H-f(o) (LuBa2Cu3O6.92, s, 298.15 K) = -2693.1 +/- 11.9 kJ/mol. The thermodynamic stability at room temperature has been assessed. The results show that Gd211 and Lu123 are thermodynamically stable with respect to binary oxides and unstable with respect to interaction with CO2 at ambient temperatures. Lu123 is thermodynamically stable with respect to assemblages containing combinations of Lu2O3, CuO, and BaCuO2 and thermodynamically unstable with respect to interactions with water.