Influences of promoter concentration (or Sm/Mg ratio), precursor for MgO (viz. Mg-acetate, Mg-carbonate and Mg-hydroxide), calcination temperature of Sm-promoted MgO catalyst on the catalytic activity/selectivity in the oxidative coupling of methane (OCM) at different temperatures (650-850 degrees C) and CH4/O-2 ratios in feed (2.0-8.0) at a high space velocity (51600 cm(3) g(-1) h(-1)) have been investigated. The catalytic activity/selectivity of Sm-MgO catalysts in the OCM are found to be strongly influenced by the Sm/Mg ratio, precursor used for MgO and catalyst calcination temperature. The catalyst with Sm/Mg ratio of 0.11, prepared using magnesium acetate and magnesium carbonate as a source of MgO and calcining at 950 degrees C, is found to be highly active and selective in the OCM process. A drastic reduction in catalytic activity/selectivity is observed when the catalyst is supported on low surface area porous catalyst carriers, indicating strong catalyst-support interactions.