Various allylbenzenes and propenylbenzenes have been hydroformylated with a 97-99% chemoselectivity using bis[(mu-acetate)( 1,5-cyclooctadiene)rhodium(I)] as a catalyst precursor. Regioselectivity of the hydroformylation can be controlled by the nature of phosphorus auxiliary ligands. The Rh-NAPHOS (2,2'-bis[(diphenylphosphino)methyl]-1, 1'-binaphthyl) system promotes the hydroformylation of allylbenzenes into linear aldehydes in near 95% selectivity and propenylbenzenes into branched aldehydes with a formyl group in alpha-position to phenyl ring in near 90% selectivity, while the Rh-dppp (1,3-bis(diphenylphosphino)propane) system gives branched aldehydes with a formyl group in P-position in near 70% selectivity starting from allylbenzenes. The regioselectivity of Rh-diphosphine systems correlates with a ligand bite angle. Both the rate and regioselectivity of the hydroformylation are largely influenced by the basicity of monophosphine auxiliary ligands, however, no correlation between their steric characteristics and the regioselectivity has been observed. (C) 2002 Elsevier Science B.V. All rights reserved.