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The mechanism of the catalytic oxidation of CO with 0, on an Mo catalyst supported on silica has been studied by a tracer technique using 1 8 0 2 . The supported Mo catalyst was prepared by using the ready reaction between Mo(q3c3H5)4 and the OH groups on silica. Extended X-ray absorption fine structure (EXAFS) spectroscopy showed that the Mo species attached to silica are dispersed atomically and have a dioxostructure. At steady state during oxidation with CO/O, = 2/1, 32% of the attached Mo species were estimated to be present as Mo6+ and 68% as Mo4+. The catalytic oxidation was accompanied by oxygen isotope exchange between 0, molecules. The oxidation of the oxostructure (Mo4+) to the dioxostructure (Mo6') by 0, molecules during the catalytic oxidation produces 0 atoms on the silica surface. Recombination of two migrating 0 atoms leads to oxygen isotope exchange between 0, molecules. The mechanism of the catalytic oxidation of CO with 0, on bulk MOO, was re-examined with reference to that elucidated on the Si0,-supported Mo catalyst.