The procedure to evaluate an oxygen chemical potential in Sm2O3-doped ceria, Ce0.8Sm0.2O2-delta (SDC), was developed using Raman spectroscopy. The oxygen chemical potentials of any positions in the SDC samples placed between fuel and air at 1273 K could be determined throughout the entire range of P(O-2) by using relative areas and intensities of their Raman spectra, Using this method, P(O-2) variation in the SDC disc-shaped sample of 4 mm in thickness and 15 mm in diameter was determined by evaluating P(O-2) between fuel- and air-side faces, and along the radius inside the pellet under the open-circuit conditions of SOFCs. In addition, the variation of total (ionic and electronic) conduction in the sample under the influence of P(O-2) gradient was discussed.