Degradation is one of the main limitations of Solid Oxide Electrolysers (SOEs) which has been frequently related to the reduction of the electrolyte at the fuel electrode side and also to anomalous high values of oxygen chemical potential, mu(O2), within the electrolyte near to the oxygen electrode side. In the present work we have studied the variation of the electrolyte reduction state across the electrolyte thickness using spectroscopic techniques when the cell is operated under electrolysis mode. The experiments have been performed in two different 10Sc1CeSZ (10 mol% of Sc(2)O(3) and 1 mol% of CeO(2) doped zirconia) electrolyte-based cells before and after electrolysis operation up to voltages of 2.8 and 1.8 V, respectively. Spatially resolved vibrational and Ce(3+) electronic micro-Raman spectroscopy and Er(3+) luminescence spectroscopy provides information about the structural changes and defect state along the electrolyte thickness. Whereas the cell remains unaltered when operating at voltages below 1.8 V, degradation was observed for the cell operated at voltages over 2 V, and the degradation is also increasing with the operation time. Different transformations in the 10Sc1CeSZ electrolyte have been observed. Crystalline structure change from cubic to rhombohedral, firstly appearing at the region near the Ni-YSZ electrode, was detected. The structure change has been further corroborated by XRD studies. The reduction of the electrolyte, as evidenced by the presence of CO, was clearly observed in the electronic micro-Raman spectra. Inhibition of Er(3+) luminescence has been associated to the presence of oxygen vacancy defects resulting from zirconia reduction. The profile of the defect distribution and structure change shows that in a thin electrolyte layer of less than 20 mu m adjacent to the oxygen electrode the electrolyte remains unaltered. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.