The reaction pathways of ethylene during methane oxidative coupling over (Li)MgO catalysts were investigated by detailed isotopic analysis of all the products from (C2H4)-C-12:(CH4)-C-13:O-2 reactant mixtures. Ethylene oxidation has little effect on the concurrent methane conversion, even at comparable mole fractions. By comparison with an extensive computer model of the gas-phase chemistry, the surface is seen to effectively quench much of the oxidative gas-phase radical chemistry. The detailed C-3 and C-4 hydrocarbon yields and isotopic distributions, however, are well described by gas-phase hydrocarbon radical pathways.