According to the results of IR characterization and catalytic study in ethylene hydroformylation, bimetallic Rh-Co catalysts can be efficiently prepared from [Rh(CO)(2)Cl](2) and cobalt carbonyls by co-impregnation on SiO2. The reaction of Co-2(CO)(8) with [Rh(CO)(2)Cl](2) (Rh:Co = 1 :3 atomic ratio) gives rapidly RhCo3(CO)(12) on the surface of SiO2. Although Co-4(CO)(12) is not reactive with [Rh(CO)(2)Cl](2) on SiO2 to form directly RhCo3(CO)(12), an equivalent bimetallic catalyst can be easily obtained from ([Rh(CO)(2)Cl](2) + Co-4(CO)(12))/SiO2 or its derivative (Rh+ + Co2+)SiO2 (Rh:Co = 1 :3 atomic ratio) under reducing conditions.