Pulse reaction method and in situ IR spectroscopy were used to characterize the active oxygen species for oxidative coupling of methane (OCM) over SrF2/Nd2O3 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 SrF2/Nd2O3. The substitution of O-18(2) isotope for O-16(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 C2H4 was observed upon interaction of CH4 with O-2-preadsorbed catalyst at 700C. 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 SrF2/Nd2O3 catalyst. (c) 2007 Elsevier B.V. All rights reserved.