In situ high-pressure IR spectroscopy studies of the rhodium catalyst systems [Rh2{μ-S(CH2)3N(Me2)}2(COD)2]/PR3 (R = Ph, OPh), [Rh2{μ-S(CH2)2S}(COD)2]/PPh3, [Rh2{μ-S(CH2)4S}(COD)2]2/PPh3, [Rh2{μ-XANTOSS}(COD)2]2/PPh3, and [Rh(acac)(CO)2]/PR3 (R = Ph, OPh) revealed the presence of mononuclear rhodium hydride species under hydroformylation conditions (80 °C, 5−30 bar). The activities and selectivities, obtained during the hydroformylation of 1-hexene using these systems as catalyst precursors, can be fully accounted for by the mononuclear species observed. Deuterioformylation experiments using dinuclear [Rh2{μ-S(CH2)3N(Me2)}2(COD)2]/PR3 systems lent no support to a dinuclear mechanism.