The inhibitory effect of two widely used quaternary ammonium compounds (QACs) - alkyl benzyl dimethyl (AB) and hexadecyl trimethyl (HD) ammonium chloride - on fermentation, methanogenesis and pentachloroaniline (PCA) dechlorination was assessed using a mixed, methanogenic, PCA-dechlorinating culture amended with AB or HD at a concentration range from 5 to 70 mu M. PCA dechlorination was inhibited at 5 mu M AB and was completely inhibited at 25 or 5 mu M by AB or HD, respectively. However, the PCA dechlorination pathway was the same in both the QACs-free and QACs-amended culture series. Fermentation (acidogenesis) and methanogenesis were inhibited by both AB and HD at and above 25 mu M but to a lesser degree than PCA dechlorination. Overall, HD resulted in a more severe inhibition of the mixed culture than AB. Adsorption of both QACs to the mixed culture biomass followed the Freundlich isotherm model. The adsorption affinity of HD for the mixed culture biomass was significantly higher than that of AB, which may be related to the observed higher inhibitory effects of HD compared to AB. Both AB and HD were not degraded in the mixed, dechlorinating culture used in this study. (C) 2013 Elsevier Ltd. All rights reserved.