Observed inhibition of methanogenesis under Fe(III)-reducing conditions is usually explained by competition of methanogens and Fe(III)-reducing bacteria for the common substrates acetate and hydrogen. However, substrate competition alone cannot explain the strong inhibition of methanogenesis during Fe(III)-reduction. We demonstrate direct inhibition of methanogenesis by amorphous Fe(OH)3 at concentrations between 0 and 10 mM in experiments with pure cultures of methanogens. The sensitivity toward Fe(III) was higher for Methanospirillum hungatei and Methanosarcina barkeri grown with H2/CO2 than for Methanosaeta concilii and Methanosarcina barkeri grown with acetate. Cultures of Methanosarcina barkeri grown with H2/CO2 and methanol demonstrated a capacity for Fe(III) reduction, which suggests that Fe(III)-reduction by methanogens may also contribute to Fe(III) inhibition of methanogenesis. Our results have important implications for kinetic modelling of microbial redox processes in anoxic soils and sediments.