A steady-state tracing kinetic study of the oxidative coupling Of methane reaction at 800-degrees-C over Li+-doped TiO2 catalyst was performed. In particular, the carbon and oxygen reaction pathways which lead to the formation of CO2 have been probed using (CH4)-C-13 and O-18(2) isotope gases under reaction conditions. The results obtained indicate that there is practically no reversibly adsorbed CH4 on the catalyst surface, while there is a very small reservoir of carbon-containing intermediate species which eventually lead to CO2 (0.1 mumol/g). A large reservoir of oxygen species, participating in the formation of CO2 (at least 12.0 mumol/g), was detected, while subsurface lattice oxygen species also participate in the oxygen reaction pathway to form CO2. In addition, large amounts of inactive carbonaceous species (17.0 mumol/g) accumulate on the catalyst surface after 1 h on stream. These species do not participate in the reaction route to form CO2 (spectator species).