BACKGROUNDIn a methane-producing bioelectrochemical system (BES) microorganisms grow on an electrode and catalyse the conversion of CO2 and electricity into methane. Theoretically, methane can be produced bioelectrochemically from CO2 via direct electron transfer or indirectly via hydrogen, acetate or formate. Understanding the electron transfer mechanisms could give insight into methods to steer the process towards higher rate. RESULTSIn this study, the electron transfer mechanisms of bioelectrochemical methane production by mixed cultures were investigated. At a cathode potential of -0.7V vs. normal hydrogen electrode (NHE), average current density was 2.9 A m(-2) cathode and average methane production rate was 1.8 mole e(-) eq m(-2) cathode per day (5.2L CH4 m(-2) cathode per day). Methane was primarily produced indirectly via hydrogen and acetate. Methods to steer towards bioelectrochemical hydrogen and acetate production to further improve the performance of a methane-producing BES are discussed. CONCLUSIONAt cathode potentials equal to or lower than -0.7V vs. NHE and using mixed cultures, methane was primarily produced indirectly via hydrogen and acetate. (Bio)electrochemical hydrogen and acetate production rate could be increased by optimizing the cathode design and by enriching the microbial community. Consequently, the production rate of CO2-neutral methane in a BES could be increased. (c) 2014 Society of Chemical Industry