The selective catalytic reduction (SCR) of N2O with CH 4 in the absence and presence of excess O2 has been studied over ion-exchanged Fe-BEA catalyst by the combination of an activity test with in-situ infrared spectroscopy to understand the nature of the surface species involved in the SCR of N2O with CH4. From the results of flow reaction studies, the Fe-BEA catalyst exhibited high activity in the SCR of N2O with CH4, even in the presence of excess oxygen, which demonstrates that the active Fe species for the SCR are formed on the ion-exchanged Fe-BEA catalyst. In the FTIR spectra of fresh catalysts, the OH band on Fe ion species (Fe-OH) was observed on the Fe-BEA catalyst, and the Fe-OH species can be involved in the SCR of N2O with CH4. Furthermore, the reaction intermediates of methoxy and formate species over the Fe-BEA catalyst were observed during the reaction. We measured the oxidation rates of these surface species with N2 and O2, and found that the methoxy species were oxidized with N2O more rapidly than O2, while the formate species were oxidized with both N2O and O2 at almost the same rate. On the basis of these results, we discuss the reaction mechanism of the SCR of N2 with CH4.