The influence of substrate loading rate on fermentative hydrogen (142) production was studied in biofilm configured sequencing batch reactor using chemical wastewater as substrate. Reactor was operated with selectively enriched anaerobic mixed microflora at different organic loading rates (OLRs; 6.3, 7.1 and 7.9 kg COD/m(3) day) after adjusting the feed to a pH of 6.0 (acidophilic) to provide suitable environment for acidogenic bacterial function. Variation in H-2 production rate was observed with change in OLR [specific hydrogen yield -13.44 mol H-2/kg CODR day (6.3 kg COD/m(3) day), 8.23 mol H-2/kg CODR day (7.1 kg COD/m(3) day) and 6.064 mol H-2/kg CODR day (7.9 kg COD/m(3) day)]. H-2 yield showed reasonably good correlation with pH drop [6.3 kg COD/m(3) day (R-2 - 0.9796), 7.1 kg COD/ m(3) day (R-2 - 0.9973), 7.9 kg COD/m(3) day (R-2 - 0.9908)]. Increase in OLR showed marked reduction in COD removal efficiency [22.6% - 6.3 kg COD/m(3) day; 19.8% - 7.1 kg COD/m(3) day and 17.2% - 7.9 kg COD/m(3) day]. (c) 2008 Elsevier Ltd. All rights reserved.