In the system Bi2O3-Er2O3, three intermediate phases were reported to date: the delta-Bi2O3 type fee phase, the hexagonal phase with layered structure, and the triclinic compound at 49 mol% Er2O3. The present study proved the hexagonal phase to be metastable; in equilibrium state around 650 degrees C, it disappears and the low-temperature stable orthorhombic phase appears at 28 mol% Er2O3, i.e., (Bi2O3)(0.72)(Er2O3)(0.28) (a = 10.7817(7) angstrom, b = 16.301(1) angstrom, c = 5.5299(5) angstrom and Z=12) which transforms to the high-temperature stable fee phase around 825 degrees C upon heating. The existence of this new orthorhombic phase implies that the fee stabilization by Er2O3 is impossible. Considerably low electrical conductivity of (Bi2O3)(0.72)(Er2O3)(0.28) causes the time-dependent degradation of the fee phase at lower temperatures on account of the extremely sluggish transformation of the fee phase to (Bi2O3)(0.72)(Er2O3)(0.28.) (c) 2005 Elsevier B.V. All rights reserved.