Two types of stoichiometric hydroformylation reactions have been described in the literature. One was performed using stoichiometric amounts of HCo(CO)(4) as the cobalt complex, and the second using stoichiometric amounts of Co-2(CO)(8) at high hydrogen pressure. It has been believed that the second reaction follows the mechanism of the first, once HCo(CO)4 was formed from Co-2(CO)(8) and hydrogen. However, there have been some serious discrepancies between the expected mechanism of this reaction and its actual experimental behavior. Using a high pressure-high temperature infrared throughput cell, we have examined in situ whether the second type reaction does indeed proceed as a stoichiometric reaction between HCo(CO)(4) and olefin, and hence, in this case, the role of Co-2(CO)(8) is merely to activate the hydrogen to form HCo(CO)4, or whether there is. some other mechanism which is more appropriate to describe the reaction. (C) 2004 Elsevier B.V. All rights reserved.