The effect on hydrogen yield of N 2 sparging was investigated in non- sterile conditions using a hydrogen-producing mixed culture previously enriched from soya bean meal. A continuous stirred-tank reactor (CSTR) at 35°C and pH 6.0 was operated on a mineral salts-glucose (10 g l -1) medium at a hydraulic retention time (HRT) of 8.5 h, and organic loading rate of 27.02 g glucose litre reactor -1 day -1. Results are reported from an 8 week period of continuous operation, and the enrichment culture gave stable results over an extended period. A hydrogen yield of 0.85 moles H 2/mole glucose consumed was obtained after 5 HRT, the gas produced being 53.4% H 2. With N 2 sparging at a flow rate approximately 15 times the hydrogen production rate, the hydrogen yield was 1.43 moles H 2/mole glucose consumed. The specific hydrogen production rate increased from 1.446 ml hydrogen min - 1g -1 biomass to 3.131 ml hydrogen min -1 g -1 biomass under sparging conditions. It is suggested that hydrogen partial pressure in the liquid phase was an important factor affecting hydrogen yield. Energy could be recovered as hydrogen from processes generating volatile fatty acids for fine chemicals and liquid bio-fuels or from acidification reactors preceding normal anaerobic biological treatment of sugary wastewaters. (C) 2000 Elsevier Science Ltd.