The oxidative coupling of methane over a La 2 O 3 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 CH 4 :O 2 :He = 13.9:2.8:83.3 (mol), temperatures from 650 to 950°C, gas space times ranging from 0.6 to 6 s and pseudo catalytic contact times from 0 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 reactionsCH 4 → CH 3 C 2 H 6 → C 2 H 5 C 2 H 4 → C 2 H 3 CH 3 → CO 2 CO → CO 2