During the catalytic partial oxidation (CPO) of methane to hydrogen and carbon monoxide distinct spatial changes of the catalyst structure in a microreactor containing a fixed-bed of 2.5 wt% Rh/Al2O3 were observed depending on temperature and space velocity. The variation of the catalyst structure along the catalyst bed was determined by X-ray absorption spectroscopy. In a first step the catalyst bed was scanned with a small X-ray beam. Then, more detailed information on the oxidation state of Rh along the catalyst bed was extracted by recording absorption images using a position sensitive X-ray camera. The studies were combined with on-line mass spectrometry providing information on the catalytic performance of the catalyst simultaneously. Mainly oxidized Rh-particles were found below the ignition temperature (310 degrees C) of the partial oxidation of methane to hydrogen and carbon monoxide. Hardly any structural changes were observed when oxidation to water and carbon dioxide occurred, but strong variations over the catalyst bed were detected when the CPO-reaction started. In the entrance zone of the catalyst bed the rhodium species were mainly in oxidized state, whereas they were in metallic state towards the end of the catalyst bed. Usually, a steep gradient within less than 100 mu m was observed. Furthermore, at higher temperature, a characteristic cone towards the inlet of the spectroscopic cell was observed. Upon temperature increase the gradient zone moved towards the inlet. The variation of the space velocity also resulted in a shift as well as a change in the shape of the profile. (c) 2006 Elsevier B.V All rights reserved.