Interestingly, the numbers obtained for the exchangeable oxygen are higher but of similar magnitude as those obtained previously for the active oxygen species that can be removed from the surface via CO pulse reduction . This might lead to the conclusion that the exchangeable oxygen species can also be removed by reaction with CO. However, control measurements performed with pure -Al2O3 revealed that active oxygen removal by CO pulsing is impossible in the absence of Ru nanoparticles. We suggest that the measured removal of active oxygen from the Ru/Al2O3 catalyst during CO pulsing is most likely due to reduction of the oxidized Ru nanoparticles, which had been fully oxidized catalyst during the oxidative pretreatment of the catalyst . This fits also to the intuitive conclusion that the very stable Al2O3 cannot be surface reduced under present reaction conditions. Hence, the exchangeable oxygen species detected by oxygen exchange with CO2 at 190 °C are very different from the oxygen species active for removal during CO pulsing at 190 °C.