A differential packed-bed reactor microfabricated in silicon is presented for heterogeneous gas phase catalyst testing. A novel cross-flow design achieves uniform flow distribution over a wide (25.5 mm) but shallow (400 mum long x 500 mum deep) catalyst bed to realize differential conversions with sufficient reaction to allow monitoring with conventional analysis techniques. The use of catalyst particles (diameters 53-71 mum) implies that conventional synthesis procedures can be used and experimental results translated to catalysts in macroscopic reactors. A set of shallow microfabricated channels maintains a spatially uniform pressure drop irrespective of variations in catalyst packing. Experiments and finite element simulations confirm the bed is isobaric with even distribution of flow. Quantitative analysis of transport effects indicates the microreactor also suppresses thermal and mass gradients in the catalyst bed. These characteristics make the cross-How microreactor a useful tool for experiments to obtain kinetics and optimize reaction conditions. Experiments with CO oxidation confirm the ability of the microreactor to obtain kinetic and mechanistic information that compare well with parameters previously determined in macroscale systems. Reactor modeling also indicates that the catalyst bed operates differentially even at total conversions that would be considered large in traditional plug flow reactors, adding to the utility of the cross-flow microreactor as a laboratory tool.