Abstract: The mechanism of oxidative carbonylation of methanol to dimethyl carbonate (DMC) over Cuβ catalyst was investigated by using in situ DRIFTS; the adsorption of single methanol, carbon monoxide and DMC as well as their mixtures on the Cuβ catalyst were considered. The results indicated that methoxide species are formed when methanol is adsorbed on the catalyst due to presence of CuO_<em>x. Only one type of active sites that are located in the six-membered ring of β zeolite is found, over which adsorbed methanol can be oxidized to methoxide and water. DMC is adsorbed on the catalyst through the contact of the oxygen atom in carbonyl group with the active sites. There were two pathways for the oxidative carbonylation: by the mono-methoxide pathway, carbon monoxide can react with mono-methoxide species to form monomethyl carbonate (MMC) and MMC then reacts with methoxide to form DMC, or by the di-methoxide pathway, DMC is formed through inserting of carbon monoxide in the di-methoxide species; latter one is more favorable over the Cuβ catalyst.