This paper presents a multiphysics model for the simulation of solid oxide fuel cell operating on biogas without external reforming. The model accounts for multicomponent species transport, heterogeneous reaction kinetics on Ni cermet anode, H2 electrochemical oxidation, and charge transport in the cermet electrodes and electrolyte. The model uses only three adjustable parameters that are fitted against one set of experimental data and then kept constant throughout the study. The model predicted voltage fluctuations are in excellent agreement with experimental measurements reported in the literature. The model also reveals that the assumption of H2 as the only electrochemically active species is probably inadequate to explain the behavior of cells operating directly on biogas. Thermodynamic analysis is further performed to support this claim made based on model predictions. The model results demonstrate that the thickness of electrochemically active layer is independent of fuel composition under fuel rich conditions, however, thins down as the fuel becomes more and more diluted. © 2015, Springer-Verlag Berlin Heidelberg.