The phase stability of 10 mol% Sc2O3 and 1 mol% CeO2 co-doped ZrO2 (10Sc1CeSZ) electrolyte during long term operation of a cathode-supported tubular-type SOFC was investigated. The surface of the cubic (c) phase of 10Sc1CeSZ electrolyte transformed to the monoclinic (m) phase via the tetragonal (t) phase during operation at 900 degrees C for 100 h. This phase transformation was facilitated by increasing the steam concentration in the inlet fuel from 3.0 to 28.5 vol.%. The amount of transformed t- and m-phases increased at longer operation times. High concentration of steam in the fuel, impurities such Mn, Al and Si in the electrolyte and the electrochemical potential gradient of oxide ion on the electrolyte surface, are thought to destabilize the c-phase. (C) 2010 Elsevier B.V. All rights reserved.