The aim of this study was to investigate the effect of hydraulic retention time (HRT) on hydrogen and methane production using a two-stage anaerobic process. Two continuously stirred tank reactors (CSTRs) were used under mesophilic conditions (37 degrees C) in order to enhance acidogenesis and methanogenesis. A mixture of pretreated ensiled sorghum, cheese whey and liquid cow manure (55:40:5, v/v/v) was used. The acidogenic reactor was operated at six different HRTs of 5, 3, 2, 1, 0.75 and 0.5 d, under controlled pH 5.5, whereas the methanogenic reactor was operated at three HRTs of 24, 16 and 12 d. The maximum H-2 productivity (2.14 L/L-R d) and maximum H-2 yield (0.70 mol H-2/mol carbohydrates consumed) were observed at 0.5 d HRT. On the other hand, the maximum CH4 production rate of 0.90 L/L-R d was achieved at HRT of 16 d, whereas at lower HRT the process appeared to be inhibited and/or overloaded. (C) 2014 Elsevier Ltd. All rights reserved.