The methane coupling reaction was investigated over MnMoO4 and alkali (Li, Na, K)-promoted MnMoO4 at 700-degrees-C using (CH4)-C-12/(CH4)-C-13 transient isotopic labeling experiments under steady-state reaction conditions. The variations observed in the surface residence times of CH4, CO, and CO2, ethane and ethylene were used to help explain selectivity differences previously reported in the oxidative coupling of methane over alkali-promoted molybdate catalysts.