Low oxygen levels (mu g O-2 L-1) in anaerobic reactors are quite common and no relevant consequences are expected. On the contrary, higher concentrations could affect the process. This work aimed to study the influence of oxygen (4.3 and 8.8 mg O-2 L-1, respectively) on the different microbial activities (hydrolytic, acidogenic and methanogenic) of thermophilic anaerobic biomass and on the methanogenic community structure. Batch tests in presence of oxygen were conducted using specific substrates for each biological activity and a blank (with minimum oxygen) was included. No effect of oxygen was observed on the hydrolytic and acidogenic activities. In contrast, the methane production rate decreased by 40% in all oxygenated batches and the development of active archaeal community was slower in presence of 8.8 mg O-2 L-1. However, despite this sensitivity of methanogens to oxygen at saturation levels, the inhibition was reversible. (C) 2016 Elsevier Ltd. All rights reserved.