Phase transformation of 1 mol% NiO doped yttria stabilized zirconia (YSZ) under SOFC operation atmospheres was analyzed by in situ Raman spectroscopy with simultaneous measurement of conductivity using a test cell consisting of air, Pt/NiO doped YSZ/Pt, wet Ar balanced 50 vol% H-2. In situ measurement was carried out under open circuit voltage (OCV) condition at 1173K for more than 20 h. Phase transformation from the cubic to the tetragonal phase progressively proceeded from the matting surface of the electrolyte on the anode side with increasing holding time. Simultaneously, the electrical conductivity of the electrolyte also exhibited monotonically decrease from about 0.103 S cm(-1) to 0.084 S cm(-1). The phase transformation started from the anode/electrolyte interface. The tetragonal to cubic phase volume ratio evaluated from the Raman spectra, rapidly decreased with increasing depth from the anode/electrolyte interface towards the center of the electrolyte. We have experimentally demonstrated by the in situ measurement that the conductivity degradation of NiO doped YSZ is closely associated to the phase transformation under a reducing condition at a high temperature. (C) 2012 Elsevier Ltd. All rights reserved.