Thermochemical investigations of 1:2:4 phases in the Ho(Dy)-Ba-Cu-O systems were performed by solution calorimetry in 6 N HCl at 323 K. On the basis of direct dissolution enthalpies of Ho2O3, Dy2O3, BaCO3, HoBa2Cu4O8, DyBa2Cu4O8 and CuO thermodynamic data were obtained for the formation of 1 : 2 : 4 phases : 1 - from the oxides; 2 - from the mixture including BaCuO2; and 3 - from the mixture including BaCO3. The data obtained showed that the abovementioned phases were thermodynamically stable with respect to the mixture with BaCuO2, namely, with respect to the reaction : 1/2Ln(2)O(3)+2CuO+2BaCuO(2)+1/4O(2)=LnBa(2)Cu(4)O(8).