This paper presents a detailed theoretical treatment of the partial oxidation of methane to syngas (OMS) on transition and coinage metal (M) catalysts (M = Ni, Pd, Pt, Cu). The adsorption energies for a number of intermediates in the dissociation of methane on the metals were calculated using medium sized cluster models of 7-13 atoms. Reaction energies for methane dissociation, syngas formation, and byproduct generation were determined. The activation energies were estimated by means of the analytic BOC-MP formula. On the basis of these results, several significant aspects involved in the OMS on the metal catalysts have been elucidated. (1) The total dissociation energy (D-e,) for the complete dissociation of methane to give surface carbon and hydrogen (CH4,5 --> C-s + 4H(s)) can be regarded as a measure for the activity of the metal in methane dissociation. The order of the calculated D-e＇s is consistent with the order of methane conversions over the metals. (2) In the presence of coadsorbed oxygen, oxygen at metal on-top site increases the adsorption energy of H and promotes methane dehydrogenation. Oxygen at the hollow site may or may not promote methane dehydrogenation, depending on the metal. (3) For the possible reactions for the coupling of the intermediates on the metal surfaces, CHx,s + CHx,s --> C2H2x,5, the trend in the calculated combination energies is in agreement with experimental observation.