Relative change in the electrogenic activity and potential losses during electron transfer in microbial fuel cell (MFC) was elucidated using real-field recalcitrant pharmaceutical wastewater as anolyte by varying substrate/organic load (OL1, 1.98; OL2, 3.96; OL3, 5.93, OL4, 7.98 kg COD/m(3)). Increase in organic load showed improvement in electrogenic activity (OL4: 346 mV, 205.61 mW/m(2); OL3: 320 mV, 158.58 mW/m(2); OL2: 290 mV, 112.87 mW/m(2); OL1: 256 mV, 72.60 mW/m(2)). Both activation and ohmic losses showed decrement with increase in organic load while concentration losses depicted increment. Improvement in power output was non-linear with increase in organic load due to the anodic over potentials, described as electron transfer resistances and potential losses. Bio-electrochemical analysis also depicted the electron losses in terms of electron discharge and energy generation with the function of varying organic load. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.