The rates of various oxygen isotope transfer processes over Li/MgO catalysts have been investigated in a flow system under conditions relevant to methane coupling. Using 16O2/18O2 mixtures, mixing to form 16O18O reached equilibrium at 700 and 750°C when helium was used as the carrier. The extent of equilibration was greatly reduced when the same mixture was used to oxidize methane. The inhibition occurs because mixing requires that the catalytic sites be in oxide form and these sites are converted to carbonate by the carbon dioxide formed from methane concurrently with ethane and ethylene. The same inhibition could be demonstrated directly when using an inert carrier by including C18O2 in the feed. When C18O2 was added during methane coupling with 16O2 as the sole oxidant, there was complete equilibration between added C18O2 and C16O2 derived from methane. In addition some 18O was transferred into unreacted 16O2. This exchange did not produce 16O2, 16O18O, and 18O2 in equilibrium. There was a slight excess of 18O2 due to the occurrence of some two-place exchange. Transient experiments in which C18O2 alone was carried over Li/MgO showed that only a small fraction of the total oxygen pool of the catalyst participated in exchange. It is probably associated with lithium. When C18O was included in reacting CH4/16O2 mixtures, approximately one-half of it was oxidized to carbon dioxide. Some exchange of 18O out of unreacted C18O took place but the rate of this process was much slower than transfer from carbon dioxide to oxygen. Calculations indicate that the catalyst is able to carry out exchange reactions of oxygen molecules faster than it reacts oxygen with methane. Hence methane coupling is unlikely to be limited by the supply of oxygen to the catalyst surface when the Li/MgO system is used.