Pulse reaction method and in situ IR spectroscopy were used to characterize the active oxygen species for oxidative coupling of methane (OCM) over SrF 2/Nd 2O 3 catalyst. It was found that OCM activity of the catalyst was very low in the absence of gas phase oxygen, which indicated that lattice oxygen species contributed little to the yield of C 2 hydrocarbons. IR band of superoxide species (O 2 −) was detected on the O 2-preadsorbed SrF 2/Nd 2O 3. The substitution of 18O 2 isotope for 16O 2 caused the IR band of O 2 − at 1128 cm −1 to shift to lower wavenumbers (1094 and 1062 cm −1), consistent with the assignment of the spectra to the O 2 − species. A good correlation between the rate of disappearance of surface O 2 − and the rate of formation of gas phase C 2H 4 was observed upon interaction of CH 4 with O 2-preadsorbed catalyst at 700 °C. The O 2 − species was also observed on the catalyst under working condition. These results suggest that O 2 − species is the active oxygen species for OCM reaction on SrF 2/Nd 2O 3 catalyst.