The effect of increasing levels of CO2, N-2, and H-2 in the feed on the homogeneous partial oxidation of methane was studied at reaction conditions giving high oxygen conversions. The reaction was carried out in a quartz lined tubular reactor at 643-703 K, 30-50 bar and residence times of 30-100 s. Air was used as the oxidizing agent and the CH4/O-2 ratio in the feed was varied from 8 to 16. Kinetic modeling studies were also carried out by using a free radical scheme consisting of a set of 46 elementary reactions. The results predicted by the model are in fair agreement with the experimental data. Increasing CO2 levels in the feed (3-15%) did not have an adverse effect on methane conversion and methanol selectivity but increased HCHO selectivity to a small extent. Increasing O-2 concentrations in the feed was found to increase the HCHO/CH3OH ratio as well as temperature required for high conversions. The need for higher temperatures is probably due to the increase in Na levels in the feed. The methane conversion of 7% and a methanol selectivity of 54% at 703 K and 34 bar obtained in this study compare well with the values reported in the literature. The model predicts that increasing H-2 levels in the feed at high oxygen conversions decreases methanol selectivity without affecting methane conversion.