The microbial interactions occurring in an anaerobic hybrid reactor which was seeded with non-sulphate-adapted sludge and subsequently exposed to sulphate over a prolonged period of 423 days were studied. The population changes upon inoculation of the reactor with sulphate-adapted sludge from a full-scale treatment plant were also analysed. Addition of sulphate (4000 mg/l) to the reactor influent resulted in a complete inhibition of syntrophic propionate degradation and severe inhibition of acetoclastic methanogenesis. No propionate or acetate-utilising sulphate-reducing bacteria (SRB) could be detected in the sulphate-fed reactor sludge. Toxicity tests carried out on sludge samples from a control reactor showed that acetoclastic methanogenesis was the most sensitive step in sulphide inhibition, with 50% inhibition observed at concentrations of 220-980 mg total sulphide/l (69-150 mg free H2S/l) over the pH range 6.5-8.0. The propionate-utilising syntrophs present in the sludge appeared to be irreversibly inhibited by sulphide and were also inhibited by acetate concentrations as low as 300 mg/l. The inoculation of fullscale sulphate-adapted sludge resulted in the establishment of propionate and hydrogen-utilising SRB in the biofilm section, but not in the granular sludge bed section, of the anaerobic hybrid reactor.