Microscale autothermal reactors remain one of the most promising technologies for efficient hydrogen generation. The typical reactor design alternates microchannels where reforming and catalytic combustion of methane occur, so that exothermic and endothermic reactions take place in close proximity. The influence of flow arrangement on the autothermal coupling of methane steam reforming and methane catalytic combustion in catalytic plate reactors is investigated. The reactor thermal behavior and performance for cocurrent and countercurrent are simulated and compared. A partial overlapping of the catalyst zones in adjacent exothermic and endothermic channels is shown to avoid both severe temperature excursions and reactor extinction. Using an innovative, optimization-based approach for determining the catalyst zone overlap, a solution is provided to the problem of determining the maximum reactor conversion within specified temperature bounds, designed to preserve reactor integrity and operational safety. (C) 2010 American Institute of Chemical Engineers AIChE J, 57: 2518-2528, 2011