Abstract The enthalpies of solution of 90K superconducting YBa 2Cu 3O 6.91 in 1N HCl aqueous solution has been measured calorimetrically at 298K. In combination with auxilliary values from literatures, the standard enthalpies of formation of YBa 2Cu 3O 6.91 has been derived. The value of Δ fH m°(YBa 2Cu 3O 6.91) is estimated 3027 ± α kJ/ mol (α is from the auxilliary values and experimental error and is less than 5). By use of entropy value of YBa 2Cu 3O 6.91. we found an increase in the stability of the compound relative to the parent oxides (Y 2O 3, BaO and CuO) from 300K up to 1156K. These results explain why the superconducting YBC oxide has a tendency to decompose above 1200K in the thermodynamic view point.