The elementary steps dealing with the methane and oxygen activation in the reaction of oxidative coupling of methane (OCM) over lanthana catalysts have been investigated by means of state-of-the-art transient kinetic techniques. Methane and oxygen activation over lanthana catalysts are shown to combine reversible and irreversible steps, parallel for methane and consecutive for oxygen. In both processes, the surface residence time of activated species per active site is found to be below the time resolution of 1 ms, characteristic of the temporal analysis of products reactor. Evidence of a strong interaction between gaseous and lattice oxygen and of an inhibiting effect of carbon dioxide on both methane and oxygen activation is also provided. Specific sites pertaining to each of these routes are proposed, accounting for the various kinetic effects which are observed under the OCM conditions.