The exact oxygen concentration of the EuBa2Cu3O7-x phase, Eu < 123 >, is not exactly known. By reducing Eu < 123 > with an argon plus hydrogen mixture in a thermobalance, the absolute oxygen concentration was obtained as well as the oxygen stoichiometry as a function of temperature and oxygen partial pressure, p(O2). Using the results obtaine, the boundary of the orthorhombic-to-tetragonal phase transition, log p(O2) (Pa) = -7194.5/T + 11.29, was derived under the assumption that this transition occurs at (7-x) = 6.5.