Full commercialisation of the solid oxide fuel cell (SOFC) technology is limited by technological challenges, in particular due to the thermomechanical reliability of the components. The long-term thermomechanical behaviour is especially affected by the operation environment. Most of the research and development studies, as well as laboratory studies are performed within furnaces rather than operating in a real system environment. However, the thermomechanical response of fuel cells operating in the furnace, especially operating with different fuel gas compositions is not fully understood, yet. The present study investigates the thermomechanical comparison of H-2, 77% CH4 and 38% CH4 operation. A three dimensional coupled CFD/FEM sub-model, derived from a whole stack model and experimentally determined data is used. The analyses account both for the thermo fluid flow and the nonlinear elastoplastic, creep strain material behaviour. The thermomechanical behavior of the most favourable operation condition has been analysed for 1200 h operation time. The results are validated qualitatively and quantitatively using postmortem images and creep strain data, respectively. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.