The oxygen reduction due to microaerophilic biofilms grown on graphite cathodes (biocathodes) in Single Chamber Microbial Fuel Cells (SCMFCs) is proved and analysed in this paper. Pt-free cathodes performances are compared with those of different platinum-loaded cathodes, before and after the biofilm growth. Membraneless SCMFCs were operating in batch-mode, filled with wastewater. A substrate (fuel) of sodium acetate (0.03 M) was periodically added and the experiment lasted more than six months. A maximum of power densities, up to 0.5 W m-2, were reached when biofilms developed on the electrodes and the cathodic potential decreased (open circuit potential of 50 – 200 mV vs. SHE). The power output was almost constant with an acetate concentration of 0.01 – 0.05 M and it fell down when the pH of the media exceeded 9.5, independently of the Pt-free/Pt-loading at the cathodes. Current densities varied in the range of 1 – 5 A m-2 (cathode area of 5 cm2). Quasi-stationary polarization curves performed with a three-electrodes configuration on cathodic and anodic electrodes showed that the anodic overpotential, more than cathodic one, may limit the current density in the SCMFCs for a long-term operation.