Ruthenium catalysts supported on various metal oxides and Co3O4- supported various transition metal catalysts prepared by an adsorption- precipitation method were examined for the aerobic oxidation of benzyl amine. We found that the Co3O4- supported ruthenium catalyst exhibited the best catalytic performance. The Co3O4- supported ruthenium catalyst was also effective for the aerobic oxidation of several other amines. The present catalyst could be used recyclably and be operated under solvent-free conditions. The activity of the present catalyst for the aerobic oxidation of benzyl amine depended less on the oxygen pressure, and the catalyst was also efficient even when air was used as an oxidant. The content of ruthenium and the size of Co3O4 particles played crucial roles in determining the catalytic performance. We have clarified that the supported hydrous ruthenium oxide ( RuO2 center dot xH(2)O) is the active species for the aerobic oxidation of benzyl amine, whereas the supported metallic ruthenium (Ru degrees) and supported ruthenium chloride ( RuCl3) are inactive, and the supported anhydrous ruthenium oxide ( RuO2) only exhibits a lower activity.