Synthesis of the YxGd1-xBa2Cu4O8 phases (x=1; 0.5; 0.75) by special method from nitrates is described in this paper. Dissolution enthalpies of YBa2Cu4O8, Y0.5Gd0.5Ba2Cu4O8, Y0.75Gd0.25Ba2Cu4O8, Y2O3, Gd2O3, CuO, BaCO3 were measured in 6 N HCl at 323 K. On the basis of obtained experimental data, the enthalpies of some reactions with YxGd1-xBa2Cu4O8 were determined. It was established that the above-mentioned 1:2:4 superconductors were thermodynamically more favourable than mixtures including CuO, YBa2Cu3Ox. It was also established that, according to the obtained data, these phases can react with CO2.