The cathode-electrolyte interface in a solid oxide fuel cell is examined to understand why premature delamination is observed in alumina substituted YSZ electrolyte. From XRD. SEM and TEM observations it was concluded that after high temperature sintering a tetragonal (Mn,Al)(3)O-4 forms at the interface, which during prolonged fuel cell operation forms a cubic (Mn,Al)(3)O-4 phase. This transformation is associated with volume decrease creating voids which ultimately weaken the cathode-electrolyte interface sufficiently for the cathode layer to delaminate off the YSZ-Al2O3 electrolyte. Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved.