This study investigated the effect of sulphate concentration and residence time on the performance of anaerobic sulphate reduction by a mixed sulphate reducing bacteria (SRB) culture using lactate as the sole carbon source and electron donor. The process performance is related to the population structure of the microbial consortia and dominant metabolic reactions. Laboratory scale chemostat cultures at different residence times (1-4 d) and sulphate concentrations (1.0-10.0 g/L) were employed. Lactate oxidation was prevalent at feed sulphate concentrations of 1.0 to 5.0 g/L. A corresponding increase in the volumetric sulphate reduction rate with increasing volumetric loading rate was also observed at this range. However, at the higher feed sulphate concentration range (10.0-15.0 g/L), sulphate inhibition, lactate fermentation and an increased microbial diversity were evident. At each feed concentration of sulphate in the range 5.0 to 15.0 g/L, varying dilution rates resulted in significant shifts in dominant metabolic reactions. Sulphate concentration and residence time have significant effects on both the structure of the microbial population and kinetics of biological sulphate reduction.