Alumina-supported rhodium particles, active for the catalytic partial oxidation (CPO) of methane, have been investigated by in situ X-ray absorption fine structure (XAFS), monitoring the gas composition simultaneously by mass spectrometry, The catalysts were prepared by anchoring of Rh-4(CO)(12) onto the support and activated by treatments in He and H-2, EXAFS analyses showed that the Rh carbonyl precursor maintained its structure during fixation, but decomposed to small metallic Rh particles upon heating in He. However, the particle size increased significantly during treatment in hydrogen, Investigations of the catalyst in the CH4/O-2 reaction mixture showed that the CPO reaction ignited at about 330 degreesC and, at the same time, the catalyst changed its structure. The metal particles disaggregated upon heating in the reaction mixture and some Rh clusters containing oxidic or carbonylic species were formed. This process was reversible with respect to temperature, The selectivity of the catalyst in the CH4/O-2 reaction mixture was significantly dependent on the residence time, However, no significant structural differences of the Rh particles have been found at different residence times. Additionally, catalyst oxidation and reduction were investigated upon O-2 and CH4 treatment.