Mixtures of light alkanes and oxygen can be converted into useful products by catalytic partial oxidation in excess fuel at moderate temperature. The relative importance of homogeneous chemistry in the reactions is still a subject of active debate, and, in this paper, the gas-phase partial oxidation of methane was simulated using detailed chemical reaction mechanisms. The sensitivity of homogeneous chemistry to the addition of trace amounts of radical and molecular species was analyzed to evaluate both the possibility of heterogeneous assistance to gas-phase reactions and the effectiveness of different species which might be added as promoters of particular product selectivities. Seeding of reaction mixtures with 2% of radicals reduced the homogeneous ignition delay time by only 50% at 1400 K. The delay was fairly insensitive to particular radical seeds because the radical pool adjusts rapidly. Seeding does not alter the product selectivity to CO and H-2 except at low conversion.