While the performance and activity of supported Ru catalysts in the CO2 hydrogenation reactions and on mechanistic aspects of these reactions were investigated in numerous studies, the interaction of CO2 with these catalysts under different reaction conditions has received much less attention so far, in spite of its crucial importance for these reactions. As part of a comprehensive study of the reduction of COx over oxide supported Ru catalysts we had recently explored the interaction of CO2 with a Ru/-Al2O3 catalyst by temporal analysis of products (TAP) reactor measurements, focusing on the removal and deposition of active oxygen (Oact) from/on this catalyst (ref. and references therein). In that study, we could demonstrate that exposing the catalyst to a multipulse sequence of CO at 190 °C can completely reduce the RuO2 nanoparticles (NPs) by removal of active oxygen (Oact) species. Based on the carbon mass balance this procedure not only led to the formation of CO2, which was released from the catalyst bed, but also to the build-up of stable carboncontaining surface species such as surface carbonates. A subsequent O2 multipulse sequence led to the formation of CO2, which was assigned to an O2-induced decomposition of surface carbonate species. Similar experiments performed with CO2 or CO2-containing gas mixtures such as CO/CO2 or H2/CO2 revealed that also these gases lead to the deposition of Oact species on the Ru surface, with the amount the amount of Oact deposition (after 1000 pulses) increasing in the order CO/CO2 < CO2 < H2/CO2  O2. To further test the interaction between CO2 and the catalyst surface, we also performed preliminary isotope labelling experiments, sending marked C 18 O2 pulses over the reduced catalyst surface. These measurements revealed significant formation of C 16 O 18 O and C 16 O2 species, indicative of oxygen exchange between C 18 O2 and the catalyst surface. Details about mechanistic aspects etc. remained, however, unresolved.