Isotopic transient techniques were applied to study oxidative coupling of methane over lanthanum oxide and strontium promoted La2O3 catalysts. Results of the O-18(2)/O-16(2) isotopic exchange experiments indicate that Sr promotion increases oxygen uptake from the lattice of the catalyst. Oxygen self diffusion coefficients, which were determined for the series of lanthana catalysts, reach a maximum for the 1% Sr/La2O3. Steps of O-18(2) in the presence of a steady flow of methane over Sr/La2O3 catalysts, indicate that surface and bulk oxygen appear in the reaction products before gas-phase O-18(2). Steps of CO2 over catalysts in which lattice oxygen has been exchanged with O-18(2), show that gas/solid exchange involves over 50% of the lattice oxygen. Under reaction conditions, methane pulses with no gas-phase oxygen yield negligible amounts of products which indicates that methane interacts with lattice oxygen only in the presence of the gas-phase oxygen.