The addition of small quantities of tetrachloromethane (TCM) into the feedstream of the partial oxidation and oxidative coupling of methane produces, with a wide variety of catalysts, advantageous effects. In general, both the conversion and selectivities to the desired products, in the latter reaction hydrocarbons containing two or more carbon atoms, are increased. Experiments in which the catalysts are pretreated with TCM and subsequently employed in the absence of feedstream TCM, together with XPS and XRD analyses show that the beneficial effects of the addition of TCM predominantly arise from the presence of chloride ions in the interfacial regions of the catalysts. Since the distribution of chloride ions within the surface is undoubtedly nonuniform, unambiguous identification of the stoichiometry is difficult, but the relatively low quantities of these ions in the surface suggest that stoichiometries similar to those of oxychlorides may be formed. Although the surface chloride ions may be participating directly or indirectly in the methane conversion process, it appears that the former is more probable.