The interaction between ruthenium and cobalt atoms in SiO2-supported catalysts prepared from various precursors by H-2 treatment at 350 degrees C has been studied by ethylene hydroformylation, temperature-programmed reduction (TPR) technique and IR spectroscopy. Incorporation of cobalt with ruthenium gives a catalyst with remarkably enhanced hydroformylation activity with respect to those of monometallic catalysts, irrespective of the ruthenium and cobalt precursors used. The synergistic effect of ruthenium and cobalt on the catalysis is consistent with TPR and IR results. TPR analysis shows regularly a promoted reduction of cobalt due to the "hydrogen spillover" effect, which indicates that ruthenium and cobalt atoms are in intimate contact in the catalysts. CO adsorption IR study demonstrates a strong decrease of CO chemisorption on Ru in the presence of cobalt, proposing that ruthenium and cobalt atoms interact on the SiO2 surface to form Ru-Co bimetallic particles. The results suggest that the catalysts thus obtained contain Ru-Co bimetallic particles, at least atoms of the two metals in intimate contact. However, in situ surface IR spectra of ethylene hydroformylation exhibit little modification by the presence of cobalt on Ru/SiO2.