Due to the synergy existing in mixed bacteria, electricity-generation performances of the mixed bacteria microbial fuel cell (m-MFC) are better than that of the pure bacteria microbial fuel cell (p-MFC). It is necessary to explore the electron transfer mechanisms of the m-MFC to further improve electricity generation. In this study, a facile "resolution-contrast" method was employed in the two-chamber m-MFC to investigate the electron transfer mechanism using mixed bacteria as the anodic inoculums. The anode was wrapped with a mixed cellulose esters membrane to eliminate the effects of direct extracellular electron transport, while the mediator electron transport was investigated by electrochemical methods. The results showed that both transfer methods existed simultaneously in the m-MFC. The main exoelectrogens were Escherichia coli, Pseudomonas aeruginosa, and Brevundimonas diminuta, and the main redox mediators included 1-hydroxyanthracene-9,10-dione, phenazine-1-carboxylic acid and 2,4-diacetyl phloroglucinol. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.