If, for example, the total amount of exchangeable 16 O and 18 O species at the Al2O3 surface shows a ratio of 3 : 1, we expect for an incoming C 18 O2 pulse that after passing through the catalyst bed the distribution of the CO2 isotopologues is C 16 O2 : C 16 O 18 O : C 18 O2 = 9 : 6 : 1, while the surface composition shows still a 16 O : 18 O ratio of approximately 3 : 1. This is not only consistent with the data in ref. , as stated also by those authors, but explains also the resulting distributions of the different CO2 isotopologues in the experiments by Guerrero et al., although they did not present such kind of quantitative analysis . In fact, knowing the amount of CO2 involved in their experiment it is also possible to derive the amount of exchangeable surface oxygen species from the measured steady-state distribution of the CO2 species. A rough estimate of this number based on the data given in that study indeed confirms this idea.