High purity H-2 production using microbial electrolysis cells (MECs) is often limited by methanogenesis. Here methanogenesis was effectively controlled by electrolytic oxygen production. Oxygen production was induced intermittently using two stainless steel electrodes, which were used as the MEC cathode during Normal operation. It was found that oxygen should be produced every 12 h or more frequently because of rapid hydrogenotrophic methanogen growth with available pure H-2. This method was also effective in an initially methanogen-dominated MEG. However, the growth of aerobic biofilms in MECs weakened methanogen control. Residual oxygen after fed-batch cycles was found to be the key indicator for effective methane control. Methane content was consistently smaller than 10% at the threshold residual oxygen volume (3 mL) or greater. MEG operation at such threshold conditions will allow high purity H-2 production, low energy consumption for O-2 production and minimal O-2 exposure on bioanodes, enabling sustainable wastewater treatment and energy recovery using MECs. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.