This paper investigates the effect of chelating agents on the bioavailability of Fe and Cu during anaerobic digestion. The results on metal speciation and methane production in anaerobic serum bottles showed that biomass was able to grow in the presence of citrate 1 mM and nitrilotriacetic acid NTA 1 mM, suggesting that the binding sites at the cell surface competed efficiently for the metals with the chelating agents added. The presence of free ethylenediaminetetraacetic acid 1 mM inhibited methanogenesis, and this seemed to be related to a loss in metal uptake capacity. Although the addition of soluble microbial products SMP did not change metal distribution in anaerobic systems, it caused an increase in the rate of methane production, and it is believed that direct uptake of Cu-SMP complexes was responsible for this increase. The best protection against Cu toxicity occurred when stoichiometric amounts of NTA, which should complex and solubilize most of the Cu, was added, and it is likely that NTA prevented lethal concentrations of Cu from being adsorbed onto the cell and hence internalized.