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The hydrogen concentration over a large palladium surface is studied by using a hydrogen sensitive palladium–oxide–semiconductor device and a scanning light pulse technique. This technique makes it possible to laterally resolve the hydrogen response of the device. Due to the water forming reaction on the catalytic palladium surface, the hydrogen concentration shows a significant decrease in the direction of a gas flow containing a mixture of hydrogen and technical air. A one-dimensional model for the hydrogen consumption in laminar flow is presented and found to compare well to measurements. The results show also that diffusion from regions beside the palladium covered part of the surface affects the response of the device. Furthermore, with the scanning light pulse technique both the local gas concentration and the local sensitivity of the device can be studied and the measurements show that local regions with a deviating sensitivity exist initially. © 1999 American Institute of Physics.