The effect of different inorganic acids viz., HNO3, HCl, H2SO4 and H3PO4 on inoculum pretreatment to selectively enrich hydrogen (H-2) producing acidogenic bacteria was evaluated in anaerobic sequencing batch bioreactors. Relative positive efficiency of HNO3 pretreated consortia in enhancing H-2 production (11.85 mol H-2/kg CODR) was noticed compared to other acids (HCl, 5.64 mol H-2/kg CODR; H2SO4, 7.65 mol H-2/kg CODR; H3PO4, 6.90 mol H-2/kg CODR) and untreated-parent consortia (control, 6.80 mol H-2/kg CODR). On the contrary, substrate degradation (COD removal) was higher with the control operation (xi(COD), 66.3%; substrate degradation rate (SDR), 1.42 kg CODR/m(3)-day) compared to pre-treated culture. HNO3 pre-treatment resulted in a shift in the fermentation pathway towards more acetic acid production, while other acid pretreatment and untreated culture showed mixed type fermentation (acetic, butyric, propionic acids). The bio-electrochemical analysis and dehydrogenase activity supported the biocatalyst performance after HNO3 pretreatment with specific enrichment of Firmicutes and Bacillus. (C) 2013 Elsevier Ltd. All rights reserved.