Bioenergy produced from co-digestion of sewage sludge (SS) and rice straw (RS) as raw materials, without pretreatment and additional nutrients, was compared for the one-stage system for producing methane (CH4) and the two-stage system for combined production of hydrogen (H-2) and CH4 in batch experiments under thermophilic conditions. In the first stage H-2 fermentation process using untreated RS with raw SS, we obtained a high H-2 yield (21 ml/g-VS) and stable H-2 content (60.9%). Direct utilization of post-H-2 fermentation residues readily produced biogas, and significantly enhanced the CH4 yield (266 ml/g-VS) with stable CH4 content (75-80%) during the second stage CH4 fermentation process. Overall, volatile solids removal (60.4%) and total bioenergy yield (8804 J/g-VS) for the two-stage system were 37.9% and 59.6% higher, respectively, than the one-stage system. The efficient production of bioenergy is believed to be due to a synergistically improved second stage process exploiting the well-digested post-H-2 generation residues over the one-stage system. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.