The aim of this work is the parametric investigation of methane steam reforming (MSR) to synthesis gas (syngas, CO + H-2) in a wall-coated catalytic microchannel reactor. Methane conversion and CO selectivity on coated Rh, Ru, Pt and Ni catalysts, all supported on Al2O3, are compared in the parameter ranges of 12.86-77.14 ms residence time, 600-800 degrees C temperature and 0.5-3.0 M steam-to-carbon ratio at the reactor inlet. Among the active metals, Rh is the best one in terms of both methane conversion and productivity (rate of methane consumption per weight of catalyst). Productivity decreases in the order of Rh > Ru > Pt approximate to Ni. For all catalysts, conversion increases with residence time, temperature and steam-to-carbon ratio. CO selectivity is highest in all cases on Rh, and increases with increasing temperature and decreasing steam-to-carbon ratio. However, in the range of residence times considered, a maximum CO selectivity for each catalyst is encountered where the water-gas shift equilibrium becomes significant, and converts more of the CO produced by MSR to CO2. Time-on-stream runs conducted on Rh and Ni show that the former has excellent chemical and mechanical stability for 72 h even at extreme conditions such as steam-to-carbon ratio of 0.5 and residence time of 12.86 ms. Activity of Ni starts to decrease after 20 h even though it is operated at a steam-to-carbon ratio of 3.0. Comparative tests conducted between microchannel and packed-bed reactors show that the former outperforms the packed bed in terms of productivity and CO selectivity. (C) 2011 Elsevier BY. All rights reserved.