Oxygen isotope exchange between C 1802 and MOO, has been examined in connection with the catalytic oxidation of C l60 with "02 on MOO,. Oxide ions at mobile and immobile sites on MOO,, which oxidize CO in the catalytic oxidation, were found to work also as active sites in the oxygen isotope exchange between C 1802 and Mo "03. The observed time dependences of the yields of C 1802, C '*O ' 6O and C ' 602 in the exchange agreed satisfactorily with those calculated from the mobile and immobile site model. In the calculation, the number of immobile sites was estimated to be 1.8% of the total surface lattice oxide ions. The exchange rate at immobile sites was 13% of the total exchange rate.