Therefore, we conclude that oxygen exchange proceeds i) at least predominantly on the support surface, without involving the Ru atoms, and that ii) it is dominated by the formation and decomposition of less stable carbonates on the Al2O3 support surface, where during bond breaking inclusion of a former surface oxygen atom or of a former CO2 oxygen atom in the newly formed CO2 molecule are equally likely. This results in a binomial distribution of the oxygen isotopes in the surface oxygen species and CO2 isotopologues.