In situ investigation of the surface composition of catalyst particles improves the understanding of the active species in heterogeneous catalysis. This study analyzed the surface chemistry of ruthenium nanoparticles supported on alumina during CO2 methanation. A model ruthenium catalyst was synthesized via DC magnetron sputtering and depositing the size selected 3.3 nm Ru-particles on an Al2O3 sample. The sample was transferred into the analyzing chamber under UHV conditions and analyzed using NAP-XPS. The emitted photoelectrons show that the main oxidation state of Ru after the deposition is RuOx, transient surface oxide. In vacuum, increasing temperature resulted in the reduction of Ru to its metallic state. The hydrogen presence in the methanation feed lowered the temperature required for the reduction from 200 °C to 100 °C. In contrast, the temperature of reduction of RuOx under CO2 atmosphere increased from 200 °C to 300 °C as CO2 is a mild oxidant. The reaction intermediates of the CO2 reduction at 350 °C consist of C–C and C–H bonds, whereas the absence of H2 leads to the formation of C=C bonds on the surface. The Ru particle size on alumina was investigated by TEM and size stability under reactive conditions was observed over the in-situ experimental timescale of hours.