Reaction of [Rh(H)(CO)(2)(BDP)] (BDP = bis(diazaphospholane)) with styrene at low temperatures enables detailed NMR characterization of four- and five-coordinate rhodium alkyl complexes [Rh(styrenyl)(CO)n(BDP)] presumed to be intermediates in rhodium-catalyzed hydroformylation. The five-coordinate acyl complexes [Rh(C(O)styrenyl)(CO)(2)(BDP)] are also observed and characterized. The equilibrium distribution of these species suggests an inversion of thermodynamic preference for branched vs linear species from the alkyl to the acyl stage.