LZ and SLZ catalysts are fairly active for oxidative coupling of methane. Their catalytic performance can be further improved by adding K+ cations (supposedly in the form of K2O) as promoters. Under conventional reaction conditions such as a reaction temperature of 1073 K, methane conversion is 18.2% and C-2-selectivity is 68.3%. The effect of adding CO2 into the feed has also been examined in order to understand the promotional effect of K2O on the reaction. XRD, CO2-TPD and XPS techniques were used to characterize the basicity and possible active oxygen species of KSLZ catalysts. The functions of K2O in KSLZ catalyst can be attributed to its high dispersion, enrichment of surface basic sites and the ability to create active oxygen adspecies on the catalyst surface. Unfortunately, at the moment it is difficult to definitely identify the nature of the active oxygen adspecies. Meanwhile, it is found that with decreasing concentrations of SrO species on the catalyst surface, the catalyst will be less sensitive to the addition of CO2 in the feed.