Solution calorimetry (6 N HCl solvent, 323 K) was used to obtain the enthalpies of certain reactions including the 247 substance (Y2Ba4Cu7O15) on the basis of solution enthalpies of Y2O3, BaCO3, CuO, Y2BaCuO5 and Y2Ba4Cu7O15. It was established that Y2Ba4Cu7O15 was thermodynamically more favourable than Y2O3+4BaCuO(2)+3CuO, Y2BaCuO5+3BaCuO(2)+3CuO mixtures at room and lower temperatures, whereas, in accordance with our earlier papers, the 123 phase in the Y-Ba-Cu-O system was metastable. This work continues our research on thermodynamic stability of superconducting phases in the Y(Gd, Ho)-Ba-Cu-O systems.