The speciation of the trace nutrients Co(II) and Ni(II) in sulfide containing media can control the methanogenic activity of Methanosarcina sp., which is of importance for the optimisation of anaerobic treatment of wastewater containing methanol. To obtain more insight in the mechanistic backgrounds, dissolved metal concentrations and methane production rates were measured for a highly enriched culture, obtained from anaerobic sludge, at various total metal and sulfide concentrations. The methanogenic activity increased with increasing total metal and decreased with increasing total sulfide. Dissolution kinetics and the influence of soluble metal sulfide species explain the trends in the concentrations of the dissolved metal species. The results suggest that the stimulation of the methanogenic activity is related to the free metal ion concentration. Flux calculations and measurements of uptake by a pure culture of Methanosarcina barkeri show that limitation by slow dissolution of the solid metal sulfides is unlikely. Growth therefore seems to be primarily controlled by the biouptake characteristics of the microorganism.