Microreactor technology creates opportunities for the development of miniature chemical devices, in which several unit operations are integrated. We describe in this paper the design, experimental, and modelling work concerning a microdevice for the preferential oxidation of carbon monoxide in hydrogen-rich reformate gas. The microdevice consists of two heat exchangers and one reactor, all integrated in a single stack of microstructured plates. Experiments show that the initial carbon monoxide conversion is high. However, the catalyst deactivates rapidly. It takes over one hour to reach the required reactor temperature during startup, which is too long for application in a portable fuel processor. The measured temperature gradients in the heat exchangers are twice as small as predicted by a one-dimensional heat exchange model of the microchannels. A two-dimensional model shows that large differences in temperature exist between channels close to the inlet and channels further from the inlet, causing the one-dimensional model to fail. This paper shows that for an accurate description of heat transfer in a micro heat exchanger, the complete (two-dimensional) plate geometry needs to be considered. (C) 2004 Elsevier B.V. All rights reserved.