The preparation of a highly active bimetallic SiO2-supported Rh-Co catalyst from RhCl3 and Co-2(CO)(8) (Rh:Co = 1:3 atomic ratio) has been studied by IR spectroscopy and ethylene hydroformylation, etc. Two steps are involved in the preparative process: (1) surface-mediated synthesis of Rh+(CO)(2)/SiO2 from calcined RhCl3/SiO2; (2) impregnation of Rh+(CO)(2)/SiO2 with a Co-2(CO)(8) solution followed by H-2 reduction at 623 K. The IR results of reductive carbonylation of calcined RhCl3/SiO2 have been compared to those of uncalcined RhCl3/SiO2 at 373 K. In situ IR observations, extraction results and elemental analysis suggest that approximately 50% of RhCl3 are transformed to Rh2O3 On the SiO2 surface and that calcined RhCl3/SiO2 is converted to a mixture of [Rh(CO)(2)Cl](2) and [Rh(CO)(2)O-s](2) (O-s: surface oxygen) under CO at 373 K. When this SiO2-supported mixture was submitted to impregnation with a Co-2(CO)(8) solution at room temperature, IR study and elemental analysis show that [Rh(CO)(2)Cl](2) reacts easily with Co-2(CO)(8) on the surface to give RhCo3(CO)(12), whereas [Rh(CO)(2)O-s](2) does not react with Co-2(CO)(8). Catalytic study in steady-state ethylene hydroformylation shows that a catalyst thus derived is more active than a catalyst derived from RhCo3(CO)(12)/SiO2 and a catalyst derived by coimpregnation of [Rh(CO)(2)Cl](2) and Co-2(CO)(8) On SiO2. This result suggests that the high rhodium dispersion of [Rh(CO)(2)O-s](2) plays a crucial role in the formation of highly dispersed bimetallic Rh-Co sites.