The partial pressure of hydrogen is an extremely important factor for hydrogen generation. This study investigated the effect of reduced pressure (via vacuum) on hydrogen production in a CSTR reactor. The results show that the reduced pressure condition is more effective in enhancing H-2 production at lower HRT (e.g., 8-4h) than at higher HRT (e.g., 12 h). The optimal hydrogen yield and overall hydrogen production efficiency occurred at a HRT of 6 h with a value of 4.50 mol H-2/mol sucrose and 56.2%, respectively. Meanwhile, at HRT 6 h the hydrogen production rate was 0.937 mol/L/d. In addition, the HPR could be further improved to 1.196 mol/L/d when the HRT was shortened to 4 h, obtaining a 37-271% increase in HPR when compared with that described in the relevant reports. For all experiments, butyrate and acetate were the two primary soluble metabolites, accounting for 85-99% of total soluble microbial products. Predominant production of acetate and butyrate demonstrates the efficient H-2 fermentation with reduced pressure processes. Copyright (c) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.