The potential energy surface for the reaction of methoxy:radical with carbon monoxide has been studied using the G2(B3LYP/MP2/CC) method. Two reaction mechanisms were revealed. The hydrogen abstraction of CH3O by CO produces CH2O + HCO via a: barrier of 24.19 kcal/mol. The addition of CH3O to CO proceeds to an intermediate CH3OCO via a barrier of 6.39 kcal/mol. The products, CH3 and CO2, can be formed in two ways. One is the C-O bond cleavage of the CH3OCO radical. The other involves the isomerization of CH3OCO to the CH3CO2 radical and the subsequent C-C bond fission. CH2O and HCO can be formed via the path CH3OCO --> TS6 --> IM4 --> TS7 --> CH2O + HCO. A radical product, CH2COOH is formed through the hydrogen rearrangement: of the CH3CO2 radical. Multichannel RRKM calculations have been carried out for the total and individual rate constants for various channels over a wide range of temperatures and pressures using the ab initio data. At lower temperatures, the title reaction is dominated by the stabilization of the CH3OCO radical. At higher temperatures, the CH3 + CO product channel and the direct hydrogen abstraction channels become dominant and competitive. The title reaction shows the typical falloff behavior. The calculations were compared with the:available experimental data.