Exchange proceeds on the support surface, most likely by reversible formation and decomposition of carbonate species by reaction with surface oxygen species, and does not involve the Ru surface atoms. It is dominated by the amount of exchangeable oxygen on the catalyst surface and in the C 18 O2 pulses. The data can be quantitatively explained in a model that assumes rapid oxygen exchange between the CO2 molecule passing through the catalyst bed and the catalyst surface, leading to equilibration between CO2 and surface oxygen isotope composition, a statistical distribution of the different exchangeable surface oxygen isotopes in the catalyst bed, and the absence of (kinetic) effects that could hinder a random population of the different isotopologues. Finally, the exchangeable oxygen species detected here differs distinctly in nature from the active oxygen that can be removed by CO pulsing under similar conditions from the fully oxidized catalyst surface. The step involving the direct binding of CO2