The influence of solids retention time (SRT) on continuous H-2 production in a submerged membrane bioreactor (MBR) was investigated using mixed mesophilic microflora. The bioreactor was continuously operated at the four SRTs of 2, 4, 12.5 and 90 d on a glucose medium under the hydraulic retention time (HRT) of 9 h and the mesophilic condition of 35 degrees C +/- 0.5. Stable biogas production with H-2 content of 50.8%-60% was achieved at SRTs ranging from 2 to 12.S d. No methane gas was observed in monitoring the experimental conditions. The H-2 production increased from 17.62 to 26.1 l-H-2/d when the SRT increased from 2 to 12.5 d, but decreased to 9.1 l-H-2/d at the 90 d SRT. The best H-2 yield, 1.19 mol-H-2/mol-glucose, was observed at the SRT of 2 d and the highest H-2 production rate, 5.81-H-2/l/ d, was obtained at the SRT of 12.5 d. Stable H-2 production was achieved by maintaining the SRT in the range of 2 - 12.5 d, regardless of the fermentative pathway related to higher lactate production. The decrease in H-2 yield was observed at long SRTs due to the low volatile suspended solids/total suspended solids (VSS/TSS) as well as the high extracellular polymeric substances (EPS) concentrations. These results suggest that the SRT is the key factor enabling sustainable H-2 fermentation in MBR, and that an SLR value of around 1.6 kg-DOC/kg-VSS/d might be the specific condition for achieving optimum H-2 production. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.