Abstract The inhibition potential of sulfide and sulfate in the methanogenesis of synthetic distillery waste has been investigated in this research with recourse to biological assay under identical laboratory conditions. Methanogenesis has been progressively inhibited with the increase of sulfide concentration in the substrate. Inhibition has been related to H 2S concentration in the evolved gas and could, accordingly, be used as an indicator of inhibition in methanogenesis. An inhibition of 50% has been observed at 5% H 2S in the evolved gas. Methanogenesis has not been inhibited by sulfate up to 5000 mg l −1 due to high substrate availability indicated by 1.33 molar ratio of TOC to sulfate. The effect of molybdate on methanogenesis in the presence of sulfate has also been investigated. The sulfate reduction and methanogenesis are both inhibited in presence of 20 mM molybdate.