Abstract During anaerobic treatment of sulphate-containing wastewaters, sulphate-reducing bacteria (SRB) compete with methane-producing bacteria (MPB) for the available electron-donors. In this work, the anaerobic treatment of a synthetic wastewater, consisting of a mixture of acetate, propionate and butyrate and high concentrations of sulphate (COD: sulphate ratio 0·5) was studied in an upflow anaerobic granular sludge bed reactor. The influence of the superficial upward liquid velocity ( v up ), the influent composition and reactor pH on the competition between SRB and MPB was investigated. At a v up of 2 m h −1 and pH 8, 93–97% of the COD was degraded by SRB. With increasing v up - values , COD removal efficiencies decreased, while at a v up of 6 m h −1 the fraction of COD removed by MPB rose to 23%. Elevation of the influent acetate concentrations, by decreasing the v up (lower recirculation) or by the use of an influent volatile fatty acid mixture with a higher acetate content, resulted in an increase of methanogenesis up to 41% of the total COD removal. In contrast, elevated levels of propionate and butyrate in the influent favoured the sulphate reducing process. A decrease of pH from 8 to 7 resulted in free hydrogen sulphide concentrations higher than 200 mg litre −1. This strongly inhibited methanogenesis while SRB were hardly affected, with a subsequent decrease of the COD removed by MPB from 41 to 7% as a result.