The ytrrium solubility in Ba–Cu–O solvent with a Ba to Cu ratio of 3 to 5 was investigated under different oxygen partial pressure [P(O2) = 2, 21, 100%]. A small amount of the solution was taken out by dipping thermally equilibrated MgO single crystals, and compositions of these specimens were analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES). The measurements were performed for the samples prepared in the temperature range from approximately 950 °C up to 1100 °C. The peritectic temperature of YBa2Cu3O72x (Y123) decreased with decreasing oxygen partial pressure. On the other hand, the Y123 liquidus lines do not show remarkable oxygen partial pressure dependency. Accordingly, the yttrium solubility at the peritectic temperature of Y123 under P(O2) = 100% was larger than those for the other conditions [P(O2) = 2% and 21%]. Assuming a regular solution, expressions for the solubility and the enthalpy of dissolution of Y123 and Y2BaCuO5 (Y211) were derived from classical thermodynamic calculations and found to be 289 kJ/mol at P(O2) = 0.02, 239 kJ/mol at P(O2) = 0.21 atm, 206 kJ/mol at P(O2) = 1 atm for Y123, and 105 kJ/mol at P(O2) = 0.02 atm, 88.0 kJ/mol at P(O2) = 0.21 atm, and 88.4 kJ/mol at P(O2) = 1 atm for Y211. Furthermore, the Jackson α factor of Y123 was estimated to be about 20, confirming a faceted growth nature of this crystal.