Carbon oxides (CO and CO2), which are generated as side products during the oxidative coupling of methane, can potentially be derived from CH4, C2H4, or C2H6 The source of COx was determined by adding (C2H6)-C-13 or (C2H4)-C-13 to the reactants and analyzing the isotopic composition of the COx. In order to eliminate the extent of reaction as a factor, a parameter, R, has been defined as the ratio of the percentage of C-13 in the COx products to that in the feed gas. If the amount of COx derived from C2H6 is small, it can be shown that R is approximately the ratio of the rate constant for COx formation from C2H4 to that from CH4. Values of R have been obtained for several oxide catalysts by introducing measured amounts of (C2H4)-C-13 into a CH4/O-2 reaction mixture and then determining the percentage of C-13 in the COx reaction product. Additional experiments, involving (C2H6)-C-13 addition to the reaction mixture, have established that the primary coupling product, C2H6, is selectively converted into C2H4; i.e., virtually no direct conversion of C2H6 into COx occurs during the methane coupling reaction. For selective oxidative coupling catalysts, the experimentally determined values of R at 700 degrees C were 2.8 for Sr/La2O3 and Ba/MgO and 4.7 for Li+/MgO. The value of R increased with increasing reaction temperature, along with the C-2 selectivity and yield, indicating that the direct conversion of CH4 into COx is less important at higher temperatures. At 850 degrees C, for example, the value of R for Ba/MgO was 6.2. It has been noted previously that, over a Li+/MgO catalyst, lower reaction temperatures favor the formation of COx from CH4. This occurs because of decreased CH4 conversion and, thus, lower partial pressures of C2H4, not because of a particularly small value of R. At 650 degrees C, in fact, the value of R for Li+/MgO was 3.4. Over a wide range of operating conditions, the rate constants for COx formation from C2H4 were several times as large as those for COx formation from CH4. This observation is consistent with the moderate C-2 yields that have been achieved during the oxidative coupling of CH4, although large values of R may also indicate that CH4 is converted into C2H6 very selectively.