The effects of adding H2O to the gas feed on the oxidative coupling of methane over Li/MgO catalyst at different partial pressures of H2O and temperatures and in the presence of cofed CO2 have been studied. Results indicated that H2O enhanced the deactivation rate. The deactivation rate increased with increasing partial pressure of the steam in the feed as well as with increasing temperature. The deactivation rate is decreased by adding small amounts of CO2 to the reaction mixture. The effects of injecting different amounts of liquid water into the catalyst bed under different reaction conditions have also been investigated. Results showed that under the conditions of our experiments water significantly enhanced the activity of the catalyst. The methane conversion increased by 86%-124% while the C2 selectivity remained relatively unchanged. The liquid water treatment significantly increased the product C2H4 : C2H6 ratio. The catalyst lithium content decreased and the BET surface area increased due to the water treatment. Some of the lithium lost from the catalyst was deposited on the walls of the reactor; however, this lithium was not responsible for the enhanced activity.