Abstract A thermodynamic phenomenological model is presented to describe thermodynamic properties, oxygen content and ordering in solid solutions from the family of phases Y 2Ba 4Cu 6+ n O 14+ n . All available thermodynamic and structural data on YBa 2Cu 3O 6+ z (123) are critically evaluated with the help of this model resulting in a self-consistent set of thermodynamic functions of the 123 tetragonal (t) and orthorhombic (o) phases. This can be used for T<100 K and for all compositions. The data phase diagram of the pseudobinary system YB 2Cu 3O 6−YBa 2Cu 3O 7 is calculated. The orthorhombic-to-tetragonal second-order phase transition curve terminates in a tricritical point where it bifurcates into two solvus lines that encircle the low-temperature two-phase (t+o) field. Thermodynamic properties of YBa 2Cu 4O 8 (124), Y 2Ba 4Cu 7O 14+ z (247) and some other phases from the homologous series Y 2Ba 4Cu 6+ n O 14+ n are estimated. The phase boundaries between the superconducting phases are calculated. They are plotted on an Ellingham and compared with data from the literature. The present thermodynamic data can be readily used for computing the conditions for thermodynamic stability of superconductors in various chemical environments.