Copper nickel, zinc and lead were added to the feed of the methanogenic stage of a two-phase thermophilic anaerobic digester which had an overall hydraulic retention rime of 10.5 h. Each metal was added for a period of 30 h. A period of 7 days was used between the metal dosing as a recovery phase. All the metals caused a reduction in the chemical oxygen demand removal which was reversed when the metal dosing ceased. Nickel and lead had the greatest impact. Lead also had the greatest effect on biogas production and, on the basis of the volatile fatty acid production appeared to act immediately on acidogenesis and move slowly on the methanogenic bacteria. Metal speciation, using a sequential extraction technique, showed that sludges in reactors being directly dosed with metals fixed the ions at sites with relatively weak binding whereas sludges in secondary digesters used sites which bound the metals strongly. Lead was an exception to this, always being bound in the relatively weak, adsorbed state.