The enthalpies of formation of Ry R’1-yBa2Cu3Ox (R, R’=Y, Nd, Eu; y=0; 0,5) have been determined at 298K by solution calorimetry. These values were shown for YBa2Cu3Ox to depend on the oxygen stoichiometry of a sample. The results of thermodynamic investigations were found to be in good agreement with the data of the kinetic investigations of the Y(Ln) Ba2 Cu3 Ox formation reactions. This agreement is explained in the aspect of electronic structure of lanthanide atoms.