Catalysts containing Ru (5 wt-%) supported on SiO₂, MCM-41, and H-Beta and H-Y zeolites have been prepared by impregnation and characterized by XPS, XAS, DRIFT, and XRD in calcined and reduced state. In the calcined state, ruthenium is present as highly disperse oxy­gen defective RuOx nanoclusters of RuO₂ short-range order on all supports except for H-Beta zeolite, where a well structured RuO₂ phase was observed. Reduction of Ru samples at 523 K in an H₂ flow leads to the formation of Ru metal particles. The dispersion of nanoparticles depends on the support na­ture. The larger Ru metal particles are formed on H-Beta zeolite, the smallest Ru⁰ clusters - on SiO₂. Ru catalysts are active (TOF = 1.8 – 4.1 h^-1) and selective (100%) in the aerobic oxidation of piperonyl alcohol to piperonal at 353 K. The turnover frequency peaks at an intermediate par­ticle size.