For the present exchange experiments, we tested whether the measured C 16 O2 : C 16 signal was chosen since here all three isotopologues are present, while at the beginning of the sequence only C 16 O2 is present. This fit results in the numbers given in Table for the relative intensities of the different CO2 species and for the 16 O : 18 O ratio in the total amount of exchangeable oxygen at these points. While correctly speaking the exchangeable oxygen includes the exchangeable surface oxygen in the Al2O3 support and the oxygen in a given CO2 pulse, it is dominated by the exchangeable surface oxygen on the catalyst, which is evident also from the much smaller pulse size. Furthermore, we also estimated the total amount of exchangeable oxygen on the catalyst from the total amount of exchanged O during the pulse sequence (Table ) and the isotope composition of the exchangeable oxygen on the catalyst at the end of the pulse sequence (Table ). The resulting values for the total amount of exchangeable oxygen, which are also listed in Table , show a decrease in the order expt. B (Figure )  expt. C (Figure ) > expt. A (Figure ). Based on the comparable C 18 O2 pulse sizes in experiments A and C and the significantly larger C 18 O2 pulse size in the experiment B, one would expect the fastest exchange for experiment B. This is partly compensated, however, by the differences in the total amount of exchangeable oxygen species present in the different experiments (B  C > A, see Table ), with the result, that the measured exchange at the end of the pulse sequence decreases in the order C > A > B (Table ). These results fully agree with the experimental findings for the increase in measured C 18 O2 pulse size with time, the decrease in measured C 16 O2 pulse size and the time required for reaching the maximum in the C 16 O 18 O pulse size.