In order to optimize H-2 and CO production from biomass gasification, the thermal decomposition of methane in a reconstituted syngas was investigated in a tubular reactor at 130 kPa, for a gas residence time of 2 s and as a function of temperature (1000-1400 degrees C), CH4 (7, 14%), H-2 (16, 32%), and H2O (15, 25, 30%) initial mole fractions. H, showed an inhibiting effect on CH4 conversion whereas H2O had few effects. Three detailed elementary mechanisms were used to predict the methane conversion rate and to identify the key reaction pathways. Flow rate analyses showed that carbon oxidation occurs mainly by addition of OH radicals on C-2 compounds. OH radicals are mainly produced by CO2 (CO2 + H = CO + OH). The inhibiting role of H-2 on CH4 conversion is explained by a competition between the OH radicals consumption channels (H-2 + OH = H2O + H). The competition between thermal conversion of methane and reforming Of unsaturated C-2 explains the soot formation.