Th0.8Ca0.2O2-delta and Na/K dispersed Th0.8Ca0.2O2-delta catalysts, synthesized by the citric acid gel method, have been tested concerning their efficiency in the oxidative coupling of methane. The structural properties of the catalysts were characterized by BET, XRD, Raman, TEM, TPD, and EPR techniques. The presence of oxide ion vacancies due to Ca ion incorporation into ThO2 lattice was evidenced by Raman spectroscopy. EPR revealed the formation of radical oxygen in the Ca-doped ThO2 sample after oxygen treatment at 500 degrees C. Th0.8Ca0.2O2-delta showed high catalytic activity reaching about 24% methane conversion (rate: 2.3 mol g(-1) h(-1)) at 56% selectivity toward Cn+ products at 600 degrees C, whose performance is comparable to that of the well known Sr doped La2O3 catalyst. Other reference catalysts like 1.9%Mn-4%Na2WO4/SiO2 and 5%Li/MgO showed considerably lower activity under these conditions. The high catalytic activity and selectivity of the Ca-doped sample is likely due to formation of stable active (O) over dot(-) species in the oxide ion vacancy site in the lattice. (C) 2010 Elsevier B.V. All rights reserved.