Fermentative hydrogen (H-2) production from a steam exploded switchgrass liquor using inhibited mixed anaerobic microbial communities was studied in upflow anaerobic sludge blanket reactors (UASBRs). Increasing the H-2 yield was accomplished by treating the inoculum with linoleic acid (LA), varying the hydraulic retention time (HRT) and sparging liquid phase with nitrogen (N-2). A maximum H-2 yield of 2.56 +/- 0.10 mol mol(-1) hexose, was obtained at a 6 h HRT in LA treated cultures sparged with N-2. Sparging or LA treatment alone was able to enhance the H-2 yield by 46 +/- 5% and 38 +/- 3%, respectively, in comparison to control cultures operating at a 6 h HRT. Of the different methods employed, N-2 sparging in combination with LA treatment proved to be more effective in enriching the H-2 producing bacteria belonging to Clostridium sp. Species belonging to Propionibacterium, Bacteroides and Eubacterium, which were associated with H-2 consumption and reduced byproducts formation, were observed in addition to Clostridium sp. in unsparged control cultures. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.