The standard Gibbs energy of formation of Bi2O3 has been measured using a solid-state cell incorporating yttria-stabilized zirconia as the electrolyte and pure oxygen as the reference electrode. An innovative cell design, with a buffer electrode to absorb the electrochemical flux of oxygen through the electrolyte caused by trace electron/hole conduction, was used to measure the oxygen chemical potential over phase mixture Bi + Bi2O3 in the temperature range from 800 to 1473 K. A closed cell was used to avoid possible deviations from equilibrium caused by vaporization of bismuth and oxides of bismuth. The results for alpha-Bi2O3 in the temperature range from 800 to 1002 K can be summarized by, Delta G(f)(o) (+/- 230)/J mol(-1) = -584235 + 289.28(T/K) For delta-Bi2O3 in the temperature range from 1002 to 1078 K: Delta G(f)(o) (+/- 230)/J mol(-1) = -553669 + 258.77(T/K) The results are discussed in relation to thermodynamic information reported in the literature. (C) 2016 Elsevier B.V. All rights reserved.