In situ microprobe Raman and XRD techniques were used to follow the oxidation state of Rh/Al2O3 and Ru/Al2O3 catalysts during the catalytic ignition process of the partial oxidation of methane (POM) to synthesis gas. It was found that the catalyst was in the fully oxidized state before ignition of the POM reaction, and abruptly changed its oxidation state at the temperature at which the POM reaction started. After the POM reaction was ignited, the amount of Rh2O3 or RuO2 in the catalyst at the entrance of the catalyst bed was below the detection level of Raman spectroscopy. Due to the greater M-O bond strength of Ru-O compared with Rh-O, Ru/Al2O3 demonstrated a greater tendency to oxidize than Rh/Al2O3 under the POM conditions. This factor affects the oxygen coverage on the two catalysts under reaction conditions and consequently affects the pathways of synthesis gas formation. (c) 2008 Elsevier Inc. All rights reserved.