A new method for the conversion of CO to H-2 was developed by anaerobic mixed culture in the current study. Higher CO consumption rate was obtained by anaerobic granular sludge (AGS) compared to waste activated sludge (WAS) at 55 degrees C and pH 7.5. However, H2 was the intermediate and CH4 was the final product. Fermentation at pH 5.5 by AGS inhibited CH4 production, while the lower CO consumption rate (50% of that at pH 7.5) and the production of acetate were found. Fermentation at pH 7.5 with the addition of chloroform achieved efficient and selective conversion of CO to H2. Stable and efficient H2 production was achieved in a continuous reactor inoculated with AGS, and gas recirculation was crucial to increase the CO conversion efficiency. Microbial community analysis showed that high abundance (44%) of unclassified sequences and low relative abundance (1%) of known CO-utilizing bacteria Desulfotomaculum were enriched in the reactor. (C) 2015 Elsevier Ltd. All rights reserved.