This paper presents a multi-physics model for simulating solid oxide fuel cells operating directly on biogas without the aid of an external reformer. The model accounts for multi-component species transport, electrochemical charge transfer and charge transport within the electrodes and electrolyte. The model uses only 3 adjustable parameters that are fitted against a base set of experimental data and then kept constant throughout the study. The model predicted voltage fluctuations due to composition fluctuation of biogas are in very good agreement with experimentally measured voltage fluctuations. The model reveals that the assumption of H2 as the only electrochemically active species is probably inadequate for explaining the behavior of cells operating directly on biogas. It is also found that the electrochemically active thickness is independent of fuel composition and the ionic current through electrolyte under fuel rich conditions. © The Electrochemical Society.