The hydroformylation of styrene was carried out in the presence of in situ rhodium, catalysts containing 2-arylphospholes with different substituents in position 2 or 3. The aryl substituents were varied from phenyl to different sterically hindered 2,4,6-trialkylphenyls. The structures and Bird-indices (BIs) of the phospholes with different steric and electronic properties were determined by DFT calculations. High chemoselectivities towards hydroformylation, as well as regioselectivities towards the branched formyl regioisomer (2-phenyl-propanal) were obtained at a temperature of 40 degreesC. Similarly, high chemoselectivity was accompanied by a decreased regioselectivity at 100degreesC. The phospholes with, an exocyclic P-function in position 2 or 3 showed higher catalytic activity. (C) 2003 Elsevier Science B.V. All rights reserved.