In this study, enthalpies of solution for Gd2O3, CuO, BaCO3 and GdBa2Cu3O6.9 phases have been measured by solution calorimetry in 6N HCl at 323 K. Experimental and literature data have been used to calculate the enthalpy of the reaction 0.5Gd(2)O(3) + CuO + 2BaCuO(2) + 0.20(2) = GdBa2Cu3O6.9. Delta(r)H was found to be -37.3 +/- 8.2 kJ mol(-1). Based on these data, we came to the conclusion that the GdBa2Cu3O6.9 phase appeared to be thermodynamically more favourable than the mixture of gadolinium oxide, copper oxide, and barium cuprate at room and lower temperatures. The experimental and literature data have allowed us to conclude that the GdBa2Cu3O6.9 phase is thermodynamically stable, whereas the YBa2Cu3O6.9 phase appears to be thermodynamically unstable at room and lower temperatures.