This study examined the decolorization ability of Caldicellulosiruptor saccharolyticus isolated from a thermal spring at the optimum growth temperature of 70 degrees C. This study demonstrated, for the first time, C saccharolyticus could effectively degrade methyl orange (MO) to 4-aminobenzenesulfonic acid (4-ABA) and N',N-dimethyl-p-phenylenediamine (DPD) with dissolved hydrogen (DH) as the reducing equivalent. The decolorization reaction was catalyzed by Ni-Fe hydrogenase and the reaction rate was correlated positively to the concentration of DH. At high concentrations of DH the decolorization rates were 6.65 and 7.08 mg/L/h, which decreased to 2.16 and 0.88 mg/L/h after purging with N-2. Furthermore, the addition of MO decreased the yield of ethanol via glucose fermentation owing to the limited reducing equivalents. We suggest the competition for hydrogen between the reduction of azo dyes and hydrogenotrophic methanogenesis processes might exist in mixed culture fermentation. (C) 2014 Elsevier B.V. All rights reserved.