A great benefit of solid oxide fuel cells (SOFC) is the possibility to use a variety of different fuels. However, feeding the anode with carbonaceous fuels can induce carbon depositions, which is a major issue for commercialization of SOFC based auxiliary power units. This work presents extensive experimental investigations of the influence of exposure to humidified methane and carbon monoxide on performance of large planar (100cm(2)) SOFC at 800 degrees C. Degradation effects are shown and explained by means of electrochemical impedance measurements (EIS), gas analyses and temperature measurements. Furthermore, distribution of relaxation time (DRT) analyses are depicted to reveal the impact of deposited carbon on mass transfer of electrochemical processes. Feeding the cells with CH4/H2O/N-2 mixtures results in better performance than when using CO/H2O/N-2 mixtures, but also induces rapid cell degradation. Methane cracking effects a carbon coverage of the Ni catalyst and ion conducting YSZ surfaces, and increases the polarization resistance. Temperature measurements indicate preferred steam reforming activity at the gas inlet region. Stable cell voltage is achieved after slight initial degradation when fueling with CO/H2O/N-2 mixtures. Carbon depositions are observed only on the surface of the anode, in the supply pipes, cell housing and the contact mesh. (C) 2017 Elsevier Ltd. All rights reserved.