The cermet of Ni-oxide ion conductor is widely used as an anode in solid oxide fuel cells (SOFCs). However, the usage of nickel induces various degradation phenomena during discharge operation; e.g., agglomeration and/or oxidation of nickel catalyst, and carbon deposition. In most cases, these degradation phenomena are triggered by the change in the oxygen chemical potential inside the anode. In this study, then, in operando Raman spectroscopy was conducted for the anode of Ni-Ce0.8Sm0.2O2-delta (Ni-SDC) cermet at 700 degrees C with applying SDC as a probe for the detection of oxygen chemical potential. This is because the band related to the oxygen vacancy in the Raman spectrum of SDC varies depending on the partial pressure of oxygen in the ambient atmosphere. The change in oxygen chemical potential at the top surface of anode was successfully quantified under polarization. The effective reaction zone of anode was also discussed by comparing with the data derived from impedance spectra measured simultaneously. (C) The Author(s) 2016. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons. org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: [email protected]. All rights reserved.