This study proposes that the on-line pH control via a model-based adaptive controller markedly improves the dark fermentative hydrogen production. According to the dynamic behavior of the dark fermentation process, pH, which rapidly declines with the beginning of the biogas production, should be precisely controlled around its optimal value in a narrow range. The success of on-line pH control was guaranteed by performing the preliminary simulation studies by experimental data obtained from dynamic analysis to determine ARMAX model order with Recursive Least Squares parameter estimation method and then to control the pH with Generalized Minimum Variance (GMV) controller. On-line control of pH at the optimal value of 6.0 during the 25 h dark fermentation process resulted in 5.4 times higher biogas production, 6.2 times higher biogas production potential, nearly doubled the duration of fermentation, and 18.4% biogas production rate increment in comparison with the uncontrolled pH case. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.