The pulse radiolysis of methane has been studied in the absence and presence of electron scavengers such as SF6 and CD3I and positive ion scavengers such as i-C4D10 in order to define the role of the intermediates H, C, CH, CH2, CH3, CH 5 + , and C 2 H 5 + in product formation. The dose rate was varied from 0.68 to 15.2 × 1019 eV/g-s, the dose (number of pulses) was varied, and the duration of the pulse was changed from 3 ns to 100 ns. The variation of the yields of the ethylene and ethane products with dose is explained by the reaction of H-atoms with accumulated ethylene product. The fast reacting C, CH, and 1CH2 species insert into methane to form acetylene, ethylene, and ethane products, but all of the reactions of these species cannot be completely specified since they may originate in upper electronic states, whose reactions with methane are unknown. Product formation by the slow reacting 3CH2 and CH3 radicals is also examined; for instance, evidence is presented for the occurrence of the reaction: 3CH2 + CH3 → C2H4 + H. Results indicate that the ions CH 5 + and C 2 H 5 + undergo neutralization mainly through the processes CH 5 + + e → CH 4 + H C 2 H 5 + + e → ( C 2 H 4 ) * + H → C 2 H 2 + H + H 2 ( 2 H ) When i-C4D10 is added, a fraction of the CH 5 + and C 2 H 5 + react with the additive rather than undergo neutralization. A calculation demonstrates that the fraction of ions undergoing reaction with a given concentration of i-C4D10 can be correctly predicted by assuming that the rate constant for neutralization of CH 5 + and C 2 H 5 + is the same as that determined recently for the t-butyl ion.