On the basis of dissolution enthalpies of Y2O3, Gd2O3, Nd2O3, Ho2O3, CUO, BaCO3, YBa2Cu3O6.9, HoBa2Cu3O6.9, GdBa2Cu3O6.9, NdBa2Cu3O6.98 measured by solution calorimetry in 6 N HCl at 323.15 K the following values were determined. (1) enthalpies of formation of Rel 2 3 phases from binary oxides: Delta(ox)H(o)(Y123, 323.15 K) = - 127.5 +/- 4.6 kJ mol(-1); Delta(ox)H(o)(Ho123, 323.15K) = -131.4 +/- 8.6 kJ mol(-1); Delta(ox)H(o)(Gd123, 323.15K) = -180.0 +/- 7.3 kJ mol(-1) Delta(ox)H(o)(Nd123, 323.15 K) = -165.0 +/- 8.8 kJ mol(-1); (2) enthalpies of formation of Rel 2 3 phases from mixtures of Re2O3, BaCO3, CuO; (3)enthalpies of formation of Rel 2 3 phases from mixtures of Re2O3, BaCuO2, CuO; (4) standard formation enthalpies of Rel 2 3 phases Delta(f)H(o)(Y123, 323.15K) = -2643.5 +/- 9.6 kJ mol(-1); Delta(f)H(o)(Ho123, 323.15 K) = -2635.3 +/- 12.2 kJ mol(-1); Delta(f)H(o)(Gd123, 323.15 K) = -2663.4 +/- 11.0 kJ mol(-1); Delta(f)H(o)(Y123, 323.15 K) = -2632.6 +/- 12.0 kJ mol(-1). The correlations between thermodynamic data and structural parameters of Rel 2 3 phases were established. It had been noted that the stability increase was correlated with the relative difference decrease of lattice parameters of Re2O3 and BaO layers from 4.65% for BaO-Ho2O3 to 4.37% for BaO-Y2O3, 3.86% for BaO-Nd2O3, 2.42% for BaO-Gd2O3.