In the ternary system Bi2O3-Er2O3-WO3, the truly stabilized delta-Bi2O3 type fcc phase has been prepared in a limited solid-solution region. This delta-phase shows good oxide-ion conduction, e.g., (Bi2O3)(0.735)(Er2O3)(0.21)(WO3)(0.055) at 550 degrees C, sigma=0.05 S cm(-1) with the activation energy=0.9 eV and the oxide-ion transport number=0.94. In contrast to "the stabilized delta-phase" reported to date in the binary system Bi2O3-Ln(2)O(3), where Ln is a lanthanoid element including Y, the present delta-phase indicates very little time-dependent degradation of electrical conductivity because of the realization of stabilization, e.g., (Bi2O3)(0.705)(Er2O3)(0.245)(WO3)(0.05) at 600 degrees C showing no degradation up to 1100 h. The present delta-phase is thermally stable up to about 1000 degrees C, since the melting onset temperature is about 1015 degrees C with negligibly small weight loss <1%. (c) 2005 Elsevier B.V. All rights reserved.