Structural studies and stability of cluster-like particles of Ru-x and RuxSey were performed in situ via X-ray diffraction at wide angles (WAXS). Both materials have a similar particle size (1.4-1.6 nm) in the reduced state. Whereas Ru-x particles are rather sensitive to oxygen from air, the chemical stabilization, against oxidation, of such metallic centers is obtained by the coordination of selenium atoms. The role of selenium atoms is also to stabilize the nanostructural nature (geometric effect) of the compound, as demonstrated by the temperature measurements (up to 300 degreesC) in the gas phase, and to provide the electronic effect for electrocatalysis.