In earlier hydroformylation studies modification of the rhodium catalyst with o-methyl-substituted or o-ethyl-substituted phosphane ligands have increased regioselectivity to branched aldehydes. The promising results achieved created a need for further studies. Hence, a wider group of o-substituted arylphosphane ligands, e.g. (2-cyclohexylphenyl)diphenylphosphane, (2-isopropylphenyl)diphenylphosphane, (2-methylnaphthyl)diphenylphosphane, (2,5-dimethylphenyl)diphenylphosphane and (2-phenylphenyl)diphenylphosphane were synthesized and tested in rhodium-catalyzed hydroformylation to support the,previous findings. Characterization of the ligands was made by NMR spectroscopy (H-1, P-31{H-1}, C-13{H-1}, HSQC, COSY-90 and COLOC). Additional parameters for evaluation of the stereoelectronic properties of the ligands were provided by quantum mechanical calculations and by synthesizing Rh(acac)(CO)(PR3) Complexes. In the rhodium-catalyzed hydroformylation of propene and 1-hexene the ligands increased the formation of branched aldehydes compared to triphenylphosphane. Additionally the increasing size of the o-alkyl-substituent was found to effect favorably to the iso-selectivity. (C) 2003 Elsevier Science B.V. All rights reserved.