The Cu(I)Y catalyst, as prepared by solid-state ion exchange at 650 degrees C, shows higher productivity and less deactivation than the known carbon-supported CuCl2 catalyst for gas-phase oxidative carbonylation of methanol to make dimethyl carbonate. The ion-exchanged Cu(II)Y showed very little activity for the same reaction. An in situ FTIR technique was applied to elucidate the reaction mechanism. The first step is the oxidation of methanol and Cu(I) to from Cu(II)-methoxide. The insertion of CO to Cu(II)-methoxide, forming probably a carbomethoxide, is the rate-limiting step. Finally, methanol and oxygen react with carbomethoxide to from dimethyl carbonate.