Anaerobic digestion of municipal solid waste (MSW) incineration leachate is limited by high organic loading rates, high concentrations of complex organic matter, and the presence of compounds like ammonia, heavy metals, and salts that can inhibit microbial activity. The purpose of this study was to investigate whether the addition of magnetite could help reactors treating MSW incineration leachate overcome these limitations and promote efficient conversion of complex organic matter to methane. These studies showed that magnetite amendments improved chemical oxygen demand (COD) removal efficiencies (78.8% vs 89.0%) and methane production rates (3.7m(STP)(3)/(m(3)center dot d) vs 4.8m(STP)(3)/(m(3)center dot d)) at an organic loading rate (OLR) of 18.2 kg COD/(m(3)center dot d). Significant differences in microbial community structures were also observed between non-amended and magnetite-amended reactors. The majority of sequences from the magnetite-amended reactor clustered with acetogenic and mixed acid fermentative bacteria. Sequences from bacteria capable of extracellular electron transfer and methanogens from the genera Methanosarcina and Methanosaeta were also more abundant in magnetite-amended reactors. It is possible that these species were participating in direct interspecies electron transfer (DIET) facilitated by magnetite amendments. In addition, ammonia nitrogen, calcium, and heavy metals were lower in effluent collected from magnetite-amended reactors than non-amended controls, suggesting that magnetite adsorbed these inhibitory compounds and created an environment more amenable to microbial growth. These results are significant and should be used to develop strategies designed to optimize bio-methanogenic treatment of complex waste with elevated contaminant concentrations.