The direct conversion of biogas to electricity using solid oxide fuel cells (SOFC) could enable a more efficient exploitation of biomass resources. However, raw biogas needs a number of pretreatment steps that include cleaning, diluting, and pre-reforming in order to protect the SOFC anode material against impurities, coking, and local cooling, respectively. Following theoretical investigations on ideal biogas dilution and reforming, SOFC single cells were operated with several gas mixtures of H2, CO, CH4, H2O, and CO2 simulating dry-reformed and steam-reformed biogas. Results showed that the SOFC performances slowly degraded during the first hours of operation before they stabilized. In a second step, hydrogen sulfide and dimethyl sulfide, two contaminants often found in biogas that may leak through the cleaning and reforming units, were added to the fuel at concentrations up to 5 ppm. Although a fast but limited drop in the operating voltage was observed, only a small fraction of these losses was irreversible and the SOFC rapidly recovered under clean conditions. Short-term and long-term expositions were studied. Online monitoring of the output voltage may serve as a good indicator of the cleaning unit state.