To learn more about the dynamic interaction between CO2 and Ru/Al2O3 catalyst surfaces and in particular about details of the oxygen exchange we furthermore investigated possible reactions by pulsing isotope marked C 18 O2 over the catalyst. In these experiments, we quantified the amount of oxygen exchange by evaluating the amount of C 18 O2 consumption and the formation of the resulting CO2 isotopologues. Additional information is derived from a comparison of the distribution of the different isotopologues with a statistical model. We are particularly interested in the effect of pre-existing surface oxygen on these processes.