Successful pH control plays a key role in hydrogen production in microbial electrolysis cells (MECs). Herein, periodic polarity reversal (PPR) is applied to a dual-cathode MEC and achieves the enhanced hydrogen production. The MEC with PPR produces 1.3 +/- 0.1 m(3) H-2 m(-3)d(-1) with 50-mM NaCl as the catholyte, much higher than 0.9 +/- 0.1 m(3) H-2 m(-3)d(-1) from the MEC with dual-working cathodes or 0.8 +/- 0.1 m(3) H-2 m(-3)d(-1) from the MEC with one working cathode. Such enhancement benefits from a slower increase in the catholyte pH, for example, it takes 15.3 h to increase the 10-mM NaCl pH from 7.00 to 12.00 in the MEC with PPR, 1.7-3.6 times that of the MECs without PPR, which is due to the decrease in the catholyte pH of the reversed cathode during PPR. The potential of the reversed electrode is more positive than the anode, suggesting that the reversed electrode acts as a second anode electrode using residue hydrogen gas as an electron source. Thus, a mechanism of in situ oxidation of hydrogen gas for pH buffering is proposed and discussed. These findings have provided a simple but effective pH control strategy for enhancing hydrogen production in MECs. (C) 2017 Elsevier B.V. All rights reserved.