Aluminum sulphate (alum) and ferric chloride are commonly employed to aid phosphorus removal in wastewater treatment. Previous studies have indicated that these chemical coagulants produce sludges that adversely affect anaerobic digestion. The objective of this study was to assess the magnitude of the effects chemical coagulants have upon methane generation in anaerobic digestion. Methane production was monitored and concentrations of aluminum or iron present during batch digestion of chemically precipitated sludge were measured.Both alum and ferric chloride addition to activated sludge produced a sludge that demonstrated reduced methane production in batch anaerobic digestion. Neither metal inhibited methanogenesis of an acetate supplement, suggesting that chemical toxicity was not a likely explanation for overall reductions in methane formation. Considering the experimental results and the findings of others, reduced methane generation is most likely caused by physical isolation of degradable substrate by the coagulant floc, which causes an overall reduction in conversion of sludge volatile solids to methane.