We have reinvestigated the phase diagram of the Eu-Ba-Cu-O system at 950degreesC in the pertinent regions of its isobaric-isothermal sections, both in air and in pure oxygen, i.e., at 0.2 and 1.0 atm oxygen pressure. The results show that there is an equilibrium quadrilateral (with O-2 as the fifth phase) which is composed of the 123, 211, 011 and 143 phases (at a specified oxygen pressure). At higher temperatures, a diagonal tie-line exists between the 123 and 143 phases rather than between the 211 and 011 phase. At temperatures lower than the 5-phase equilibrium the tie-line connects the 211 and 011 phases. The equilibria change as a function of temperature has been studied hereby, and the resulting modification of the Eu-Ba-Cu-O phase diagram as a function of temperature and O-2 pressure is reported. The observed new high-temperature phase relationship allows the "143" phase to be considered instead of 011 as a possible pinning center in 123. This may have important implications with respect to raising the critical current density, J(c), of the EuBa2Cu3O7-delta high-T-c superconductor. (C) 2003 Elsevier Science B.V. All rights reserved.