The sulfate effects on fermentative hydrogen production from sucrose were studied using enriched mixed-microflora and continuous reactor operations. The tested sulfate concentrations ranged from 500 to 3000mg SO42-/l at two pH values of 6.7 and 5.5. The experimental results indicate increased sulfate inhibited hydrogen production by changing the microbial metabolic pathway from a butyrate to ethanol fermentation. This inhibition recovered in a sag curve way and its recovery pattern model was developed. The inhibited hydrogen production at 3000mg SO42-/l and pH 6.7 diminished entirely by lowering the operating pH to 5.5, at which sulfate-reducing bacteria could not survive. Active hydrogen producing bacteria now dominated. resulting in about a 40% increase in hydrogen production relative to the control with a hydrogen yield of 3.6 mol-H-2/mol-sucrose. hydrogen production rate 374 mmol H-2/l/day and specific hydrogen production rate 90 mmol H-2/g VSS/day. In the biogas. the H2S concentrations were lower than 100 ppm, even at 3500 mg SO42-/l (pH 6.7) and was < 0.1 ppm when pH was 5.5. (c) 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.