Micromachining of chemical reactors enables the manufacture of microchannel reactors with unsually large surface-to-volume ratios. This can strongly affect the coupling between heterogeneous (wall) reactions and homogeneous (gas-phase) reactions, ultimately leading to a complete quenching of homogeneous reactions. Using a 2D boundary-layer model coupled with detailed reaction kinetics for surface and gas-phase reactions, we investigate the ignition behavior of hydrogen/air mixtures in a Pt-coated microchannel. The influence of temperature, pressure, reactor diameter, and fuel-to-air ratio is studied. We find that a purely kinetic radical scavenging by the catalytic surface can indeed result in a complete suppression of gas-phase reactions. However, the attainability of "intrinsic safety" in microchannel reactors is strongly dependent on a fine interplay between homogeneous and heterogeneous reaction pathways in the individual reaction system. In particular, the strong dependency of homogeneous reactions on pressure leads to a breakdown of intrinsic reactor safety at sufficiently high reactor pressure. A generalized equation for the boundary of safe reactor operation is derived for the current reaction system. (c) 2006 American Institute of Chemical Engineers