[en] Highlights: • A novel in-situ XPS study was performed on plasma-deposited Ru-based catalytic films. • The content of various Ru oxidation states can be controlled by the Ox/Red processes. • The key role of temperature in changes between Ru oxidation states is demonstrated. - Abstract: A novel in-situ study of the surface molecular structure of catalytically active ruthenium-based films subjected to the oxidation (in oxygen) and reduction (in hydrogen) was performed in a Cat-Cell reactor combined with a XPS spectrometer. The films were produced by the plasma deposition method (PEMOCVD). It was found that the films contained ruthenium at different oxidation states: metallic (Ru⁰), RuO₂ (Ru⁺⁴), and other RuO_x (Ru^+x), of which content could be changed by the oxidation or reduction, depending on the process temperature. These results allow to predict the behavior of the Ru-based catalysts in different redox environments.