Biogas produced during anaerobic treatment of sulphate containing wastes, such as distillery waste, invariably contains around 1-3% (v/v) H2S. Being highly corrosive, H2S reduces the utility of biogas as a fuel for boilers and also makes the generation of electricity from biogas very problematic. Sodium molybdate is known to inhibit sulphate reduction and H2S production. To evaluate the efficacy of molybdate during anaerobic digestion of distillery waste, a single dose of 3 mM sodium molybdate was given to the well stabilized anaerobic digester. The digester was run at the loading rate of 1.33 kg COD m(-3) d(-1) on the distillery waste which had a sulphate content of 10 g l(-1). A single dose of 3 mM molybdate inhibited production of H2S for 9 days. Continuous dosing of 3 mM molybdate inhibited H2S production for only 11 days after which H2S was again produced, while methane production declined from day 66. The inhibitor was bacteriostatic to both SRB and acetoclastic methanogens, but more toxic to SRB. It did not inhibit hydrolytic bacteria.