The kinetic isotope effect for CH4 compared to that for CD4 has been measured for the oxidative coupling reaction of methane over Li MgO, SrCO3, and Sm2O3 catalysts in a flow reactor. Each catalyst gave results consistent with CH bond breaking being the slow step. For temperatures between 680-780 °C over Li MgO, kH KD decreased slightly with temperature. The isotope effect for ethane production was more sensitive to the level of conversion and declined from 1.8 at low conversion to near unity under conditions where the ethylene to ethane ratio was high (~1). Selectivities to hydrocarbons were lower with CD4 and did not change with decreased flow rates, implying that either COx and C2 products arise by totally separate slow steps or, if a common step with CH3 radicals is involved, then COx formation occurs on the catalyst. Experiments with CH4 CD4 mixtures showed that CH3CD3 and CH2CD2 were the dominant mixed products. The distribution of the ethanes always reflected the relative concentrations of CH3 and CD3 determined by the kinetic isotope effect. At low ethylene to total C2 ratios (~0.2) this was also true for ethylene; but at higher ratios substantial exchange to produce ethylenes other than C2H4, CH2CD2, and C2D4 occurred. The concentration of the exchanged methanes correlated with total methane conversion but was dependent on the surface. Exchange in the ethylenes also correlated with exchange in the methanes and purely gas phase processes appear at least partially responsible. H2: HD: D2 ratios are always at equilibrium and exchange also occurs between CD4 and H2.