The formation of RhI(CO)2 from Rh6(CO)16 impregnated on SiO2 and its reactivity toward C2H4/H-2 have been investigated by in situ infrared spectroscopy. impregnation of Rh6(CO)16 on partially dehydroxylated SiO2 results in a partially decarbonylated structure exhibiting the terminal and bridging carbonyl bands at 303 K; surface H2O reacts with Rh6(CO)16 to form RhI(CO)2 at 373-423 K. The terminal carbonyls show no reactivity toward C2H4/H-2 at 303 K while they begin to react with C2H4/H-2 to produce adsorbed acyl and propionaldehyde at 323 K. Under the same condition, the terminal carbonyls are less active for CO insertion than gem-dicarbonyl derived from Rh6(CO)16 and linear CO on the surface of Rh crystallites. The low reactivity of the terminal carbonyls compared to that of linear CO may be attributed to the lack of neighboring Rh sites for adsorbed alkyl, C2H4 and hydrogen. Steady-state ethylene hydroformylation reveals that the Rh is in an oxidized state and that hydrogenation of acyl species is rate limiting at 393 K and 1 MPa. Increasing reaction temperature to 513 K leads to the reduction of oxidized Rh sites, hydrogenation of adsorbed acyl, and an enhancement of propionaldehyde selectivity.