Iron-catalyzed formation of filamentous carbon from methanol was studied at 500-600-degrees-C by exposing iron wire to methanol partial pressures of 1.33-300 kPa in the presence of added partial pressures of water vapor, carbon dioxide, and hydrogen. Filamentous carbon formed from feedstreams containing only methanol, and this process was accompanied by metal dusting corrosion. Addition of a sufficient water vapor partial pressure prevented both carbon deposition and corrosion. The role of water vapor is viewed as being to maintain the iron surface in an oxide state, which is inactive for the catalytic formation of filamentous carbon. Carbon dioxide was found to be a much less effective oxidant than water vapor, while hydrogen did not prevent filamentous carbon formation. Carbon deposition from methanol occurs under conditions where it would not be expected from the equilibrium products of its gas-phase decomposition (CO, CO2, H2O, H-2, and CH4).