The influence of reduction and re-oxidation cycles on microstructure and residual stress of anode supported planar solid oxide fuel cells (SOFCs) has been studied. Shrinkage caused by the reduction of NiO to Ni is not reversible. Microstructural scanning electron microscope observations reveal that after re-oxidation, the NiO has a higher porosity and requires a larger volume than in the initial, oxidised state. As a consequence of the anode expansion, tensile stresses develop in the electrolyte, ultimately causing fracture. (c) 2005 Elsevier B.V. All rights reserved.