6. The relative pulse intensities of the different CO2 isotopologues at different stages of the pulse sequence can be rationalized by a model, which assumes rapid oxygen exchange between exchangeable surface oxygen at the catalyst surface and CO2. This leads to rapid equilibration of the 18 O: 16 O ratio in the exchangeable surface oxygen and in the CO2 gas phase molecules. The observation of a binomial distribution of the different emitted CO2 isotopologues, which is determined by the isotope composition of the total amount of exchangeable oxygen on the catalyst surface and in the CO2 pulse, indicates a statistical distribution of the different exchangeable surface oxygen isotopes in the catalyst surface and in the CO2 molecules. The absence of (kinetic) effects that could hinder a random formation of the different isotopologues. Hence, decomposition of the surface carbonates must proceed via a process in which breaking of a Al - 16 O bond (with former surface lattice oxygen) is equally likely as breaking a Al - 18 O bond (with 18 O from the former C 18 O2 molecule.