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High purity H2 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 hours or more frequently because of rapid hydrogenotrophic methanogen growth with available pure H2. This method was also effective in an initially methanogen-dominated MEC. 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. MEC operation at such threshold conditions will allow high purity H2 production, low energy consumption for O2 production and minimal O2 exposure on bioanodes, enabling sustainable wastewater treatment and energy recovery using MECs. Keywords Hydrogenotrophic methanogens; oxygen evolution by electrolysis; oxygen threshold for methane control; bioelectrochemical system (BES); high purity hydrogen gas (H2) production; sustainable wastewater treatment 1