The deactivation mechanism for the hydroformylation of 1-hexene was studied by cylindrical internal reflectance infrared spectroscopy (CIR-FTIR) under autogenous conditions. These studies showed that the initial deactivation involved the conversion of the most active catalytic intermediate, RhH(CO) 2(PR 3) 2, 1, to a less active orange dimer, [Rh(CO)(PR 3) 2] 2, 2, which was followed by formation of a totally inactive binuclear complex having a bridged phosphide ligand. The in situ spectra showed that when the first two complexes were observable in solution hydroformylation was facile, and when they were replaced by the phosphido complex all activity stopped. A series of hydroformylation reactions was carried out in which the triphenylphosphine ligands carried a variety of para-substituents on the phenyl rings. Deactivation was rapid when the substituents were strongly electron-donating, such as p-methoxy and p-dimethylamino.