The paper presents results of systematic studies in the effects of a wide range of amounts of chloride ions introduced to the Na/CaO catalyst on the oxidative coupling of methane carried out in varied reaction conditions concerning a wide range of temperatures, oxygen concentrations and contact times of reagents with the catalyst. The positive influence of the presence of chloride ions is evident at their relatively small contents in the catalyst and specific effects depend mainly on the reaction temperature. There is an optimal content (about 2 wt.%) of chloride ions in Na/CaO. at which the total selectivity of reaction and selectivity of ethane formation reach the highest values, higher than those with the chloride-free catalyst. An improvement in the selectivity of ethylene formation is obtained only at higher reaction temperatures. Still, at every amount of chloride ions present in the catalyst, there may occur a decrease in its activity, especially at lower reaction temperatures. The operation of the chloride modifier consists in poisoning the catalyst surface and diminishing the number of sites with active oxygen, which results in excluding primarily the most active of these sites from methane transformations. (C) 2007 Elsevier B.V. All rights reserved.