Microbial fuel cells (MFCs) are bioelectrochemical devices that directly convert the chemical energy stored in complex organic compounds into electricity. In order to improve their performance, a complete understanding of the reactions that occur inside bioanodes is essential. Electrochemical Impedance Spectroscopy (EIS) represents a powerful method for the investigation of these biophysicochemical processes. However, the larger part of EIS studies on MFCs proposed in the literature are often misconducted, employing cell configurations or measurement conditions which do not always allow obtaining reliable conclusions. In this work, EIS measurements, in three electrode configuration, were recorded as a function of different external resistances, to characterize and compare the impedance response of two different mixed-community microbial bioanodes coming from the same freshwater sediment sample but enriched with two different media, namely a general one and a ferric citrate one. In order to construct an appropriate equivalent electrical model and to give a right physical interpretation of the obtained results, focused control experiments in abiotic and starving conditions were performed. Three processes were found to govern the bioanode impedance: a substrate/products diffusion process, a biofilm process and an electrode/electrolyte interfacial process. General-enriched bioanodes showed a more efficient electron transfer mechanism, with faster biofilm and diffusion time constants, lower resistance values and higher charge storage capacities. The maximum power point of general-enriched biofilms (79 mW/m(2)) occurred at an anode potential of -0.4 V; on the contrary ferric citrate enrichment resulted in a maximum power density of 38 mW/m(2). These results were in accordance with cyclic voltammetry analysis, that demonstrated a charge transfer mechanism centered at -0.4 V for general enrichment and -0.22 V for ferric citrate one. (C) 2017 Elsevier Ltd. All rights reserved.