Microreactor technology creates opportunities for the development of miniature chemical devices, in which several unit operations are integrated. In this paper, we describe the design, model simulation, and experimental validation and operation of a microdevice for the preferential oxidation of carbon monoxide in hydrogen-rich reformate gas. The microdevice integrates two heat exchangers and one reactor, all consisting of welded stacks of microstructured plates. We show that the device is able to reduce the carbon monoxide concentration to 10 ppm in combination with a high heat recovery efficiency of 90%. The design of the microstructured plates was based on a combination of three-dimensional computational fluid dynamics simulations of the flow pattern and a two-dimensional heat transfer model, in which the flow pattern was approximated. An integral one-dimensional micro heat exchange model was then used to design the integrated device. This approach may be well suited for designing microstructured reaction systems. (C) 2004 Elsevier Ltd. All rights reserved.