This paper investigates if it is possible to produce inocula to counteract high acetic acid (CH3COO ) concentrations during thermophilic anaerobic digestion. To this end, fermenter sludge was exposed for different durations to either gradually increasing CH3COO concentrations or directly exposed to a high concentration (150 mM). Altogether, these enrichments led to inocula with a distinct decrease of representatives of Methanobacteriales, while those of Methanoculleus spp. were hardly affected by any treatment. After the inoculation, good agreements of the abundance of Methanosarcinales and Methanoculleus spp. with total DNA content and methane production rate were apparent. In addition, a gradual adaptation of the inoculum for at least 4 weeks led to a significant increase of Methanosarcina spp. during the subsequent fermentation. These results demonstrate the potential of bioaugmentation to relieve the start-up of an anaerobic digestion suffering from high CH3COO loads, especially pointing to the robust acetoclastic genus Methanosarcina. (C) 2013 Elsevier Ltd. All rights reserved.