Catalytic hydroformylation of olefins has been carried out in a HP FT-IR cell using RhH(CO)(PPh3)(3) catalytic precursor. A different behaviour was noticed between a terminal (hex-1-ene) and an internal alkene (cyclohexene) and different rate-determining steps of the catalytic cycle have been hypothesised. The hydroformylation of hex-1-ene has also been tested in the presence of Co-2(CO)(S) as catalyst. In this case, only the catalytic precursor is evidenced by HP FT-IR. Finally, the influence of an additional gas (helium. nitrogen or argon) in the reaction medium was evaluated: a high pressure of argon or nitrogen affects the initial rate of the reaction as shown by a decrease of the rate of the aldehyde formation. (C) 2004 Elsevier B.V. All rights reserved.