To study the heterogeneous steps of the oxidative coupling of methane to ethane and ethene over MgO, Li/MgO and Sn/Li/MgO, oxygen and carbon dioxide isotope step experiments were carried out in the absence of reaction, and oxygen and methane isotope step experiments were carried out in a tubular reactor at 1023 K, atmospheric pressure, an inlet molar ratio of CH4/O-2 equal to 4, a methane conversion of 24%, and an oxygen conversion of 85%. The steady-state axial total concentration profiles of the reactants, intermediates, and products have a significant influence on the shapes of the transient isotope responses under these conditions. Oxygen interacts strongly with all catalysts used by dissociative reversible adsorption, except for lined-out Li/MgO. Both surface and bulk lattice oxygen participate in the reaction. The promotion with lithium and even more with tin increases the mobility of oxygen in the bulk of the catalyst and the amount of exchangeable oxygen per unit BET surface area. Carbon in methane can either react to C-2 products, without any significant interaction with the catalyst, or show a weak reversible interaction with the catalyst, which does not lead to C-2 products. In the absence of reaction, carbon dioxide interacts with the catalyst only in the presence of lithium. Under reaction conditions, the experiments can be described satisfactorily by postulating a methoxy species as the only carbon-containing intermediate on the catalyst leading to carbon dioxide.