The catalytic transformation of CH4 in the presence of O-2 over La2O3, MoO3/HZSM-5 and the mixed catalyst, La2O3 + MoO3/HZSM-5, has been investigated and compared. MoO3/HZSM-5, which is known as an active catalyst for the dehydrogenation and aromatization of methane in the absence of oxygen, shows very low selectivity towards the formation of aromatic compounds in the presence of oxygen. However, the combination of MoO3/HZSM-5 with La2O3 which is a very active catalyst for the production of ethane, ethylene, CH3 and CH2 species under the reaction conditions, provides a catalyst, La2O3 + MoO3/HZSM-5, that is rather active for transformation of methane to styrene. At 1023 K, a methane conversion of 18.3% and styrene selectivity of 10.8% are obtained from a CH4/O-2 (6/1 mole ratio) reactant gas in a feed flow rate of 3500 ml/h. Normal deuterium isotope effects on both the methane conversion reaction and the styrene formation over the La2O3 + MoO3/HZSM-5 catalyst have been observed in the reaction.