The effects of linoleic acid (LA (C18:2)) and its degradation by-products on hydrogen (H-2) production were examined at 37 degrees C and an initial pH value of 5.0 using granular and flocculated mixed anaerobic cultures from the same source. In the flocculated cultures, the H-2 consumers were inhibited to a greater extent when compared to the granular cultures. The maximum H-2 yields were 2.52 +/- 0.2 and 1.9 +/- 0.2 mol mol(-1) glucose in the flocculated and granular cultures, respectively. The major long chain fatty acids (LCFAs) detected at which H-2 attained a maximum value were LA (750 mg L-1) and myristic acid (MA) (500 mg L-1). Bacteroides sp. dominated the granular cultures whereas Clostridium sp. and Bacillus spp. were the major microorganisms in the flocculated cultures. A flux balance analysis (FBA) revealed that homoacetogens consumed more H-2 in the granular when compared to the flocculated culture. A principal component analyses (PCA) showed the first three principal components accounted for approximately 76% and 68.5% of the total variability for the LA and its degradation by-products and the T-RFLP data sets, respectively. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.