Nowadays, microbial fuel cells (MFC) stand up as a promising renewable energy source. Due to the ability of the MFC to oxidize a wide spectrum of substrates, wastewater seems to be one of the most interesting fuels. Unfortunately, wastewater could contain electron acceptors such as nitrate, which could interfere with the electrical performance of the MFC. In this work, the influence of oxidised nitrogen forms on the electricity production of an air breathing MFC was studied. It was observed that power generation was not significantly influenced when operated at nitrate concentrations below 0.9 mg N-NO3 L-1, However, when the nitrate concentrations were above this value, the electricity generation was reduced. In order to investigate the causes of this reduction, an electrochemical characterization of the MFC was performed. The results of polarization curves were fitted to an empirical mathematical model. From the results it was observed that, when nitrate was present, the denitrifiers outcompeted the electrogenic microorganisms for substrate, causing mass transfer limitations to the electrogenic process.