The objective of this research was to clarify the internal resistance of the PMFC. To characterize internal resistances of the PMFC current interrupt and polarization were used, and partial resistances were calculated. The internal resistance consisted mainly of anode resistance and membrane resistance which both decreased during current interrupt. The anode resistance was the result of mass transfer resistance in the electrochemically active biofilm. The membrane resistance was the result of accumulation of cations in the cathode. The polarization showed a distinct hysteresis which was explained by the increase of the internal resistance during polarization. The increase of this resistance makes it difficult to interpret the maximum power output of the PMFC. (C) 2012 Elsevier Ltd. All rights reserved.