The oxidative coupling of methane over a La2O3 catalyst was investigated by means of a continuous flow reactor with a perfectly mixed gas phase and a variable number of catalyst pellets. Experiments were carried out isothermally, i.e. at the same temperature for the gas phase and the catalyst, under the following conditions : total pressure equal to 1.05 bar, inlet mixture CH4:O-2:He = 13.9:2.8:83.3 (mol), temperatures from 650 to 950 degrees C, gas space times ranging from 0.6 to 6 s and pseudo catalytic contact times from O to 60 mg s cm(-3). Complementary experiments were performed on carbon monoxide oxidation. Our results are in favour of a reaction mechanism involving, besides gas phase free radical elementary reactions, surface initiation reactions CH4 --> . CH3 C2H6 --> . C2H5 C2H4 --> . C2H3 and surface oxidation reactions . CH3 --> CO2 CO --> CO2