The EMF of the following galvanic cellsW, Bi, Bi2O3\CSZ\O-2 (1 atm), RuO2, Pt (I)W, Bi, Bi2O3\CSZ\O-2 (1 atm), Pt (II)were measured over a wide range of temperature (572 to 1127) K. With 02 (1 atm) reference and RuO2 as the electrode material with Pt as electrical lead, measurements were possible at temperatures close to the melting temperature of Bi. Standard Gibbs free energies of formation, Delta(f)G(m)(0) of Bi2O3(S) (both alpha and delta phases) and Bi2O3(l) were calculated from the emf measurements and are given by the following expressions:Delta(f)G(m)(0)(alpha-Bi2O3) +/- 0.3 kJ (.) mol(-1) = -583.4 + 0.2938T/K (572 less than or equal to T less than or equal to 988 K),Delta(f)G(m)(0)(delta-Bi2O3) +/- 0.4 kJ (.) mol(-1) = -543.8 + 0.2538T/K (988 less than or equal to T less than or equal to 1098 K),Delta(f)G(m)(0) (Bi2O3) +/- 0.3. kJ (.) mol(-1) = -492.4 + 0.2070T/K (1098 less than or equal to T less than or equal to 1127 K).A third law treatment of the data yielded a value of -568.4 +/- 0.5 kJ (.) mol(-1) for the enthalpy of formation of Bi2O3(s) at T = 298.15 K, Delta(f)H(m)(298.15 K)(0)(alpha-Bi2O3), which is in good agreement with the second law estimate of -570.8 +/- 0.3 kJ (.) mol(-1). (C) 2003 Elsevier Ltd. All rights reserved.