The profile of oxygen chemical potential in 20 mol% Sm-doped ceria Ce0.8Sm0.2O2-delta (SDC) at 1273 K under the open-circuit conditions of solid oxide fuel cells was determined using Raman spectroscopy. SDC pellets were annealed in various atmospheres, and a peak intensity of a Raman band of SDC in the range 540-600 cm(-1), which has been assigned to a band that originates in the oxygen vacancies, increased with decreasing oxygen partial pressure [P(O-2)] upon annealing. A clear relationship between the peak area of the oxygen-vacancy band and P(O-2) upon annealing was obtained. This relationship was used to determine the profile of oxygen partial pressure, P(O-2), in SDC located between fuel and air. The experimental profile was in good agreement with that obtained theoretically. The profile revealed that most part of SDC was mixed conductive and only a thin layer adjacent to the air side remained purely ionic conductive under the open-circuit conditions of SOFCs. (C) 2000 Elsevier Science B.V. All rights reserved.