A four-layer solid oxide fuel cell stack with planar anode-supported cells was operated galvanostatically at 700 degrees C and 0.5Acm(-2) for nearly 35,000 h. One of the four planes started to degrade more rapidly after 28,000 h and finally more progressively after 33,000 h. The stack was then shut down and a post-test analysis was carefully performed. The cell was characterized with respect to cathodic impurities and clarification of the reason(s) for failure. Wet chemical analysis revealed very low chromium incorporation into the cathode. However, SEM and TEM observations on polished and fractured surfaces showed catastrophic failure in the degraded layer. The cathode barrier electrolyte cell layer system delaminated from the entire cell over large areas. The source of delamination was the formation of a porous, sponge-like secondary phase consisting of zirconia, yttria and manganese (oxide). Large secondary phase islands grew from the electrolyte anode interface towards the anode and cracked the bonding between both layers. The manganese originated from the contact or protection layers used on the air side. This stack result shows that volatile species in this case manganese should be avoided, especially when long-term applications are envisaged.