Enhancing hydrogen (H-2) production by inhibiting or controlling the growth of H-2 consuming microorganisms can be accomplished by thermal and chemical methods as well as engineering control strategies. In this study, H-2 production from a mixed anaerobic community was evaluated after applying heat shock, acid treatment, alkaline treatment or adding linoleic acid (LA). The H-2 yield trend based on the Tukey's test was as follows: LA congruent to acid congruent to heat shock congruent to alkali > control. In comparison to the control cultures (without any treatment), methanogenic activity was not detected in cultures, which were heat-shocked and chemically treated with acid or LA. However, methanogenic activity was observed in cultures treated with alkali. Lactic acid was observed in controls (720 mg L-1) and cultures treated with alkali (600 mg L-1). Even though acetic acid production was dominant in cultures treated with alkali, the H-2 yield was not the greatest in this culture. Based on the PCA, the microbial populations in the LA and heat treated cultures were more similar when compared to the other treatment methods. Note in the case of LA, the maximum H-2 yield was likely a result of a greater inhibitory effect imposed on the different H-2 consuming populations. (C) 2012 Elsevier Ltd. All rights reserved.