A series of triphenylphosphines modified with different heteroatom groups (-SCH3. -N(CH3)(2), -OCH3, -CF3) in ortho or para position of the phenyl ring(s) were synthesised and tested for their catalytic behaviour in the rhodium catalysed hydroformylation of l-hexene (80 degreesC, 15 bar) and propene (100 degreesC, 10 bar). Hydroformylation results for 1-hexene and propene differed markedly. With 1-hexene, the differences in activity and in chemo- and regioselectivity obtained with the various ligands were minor. Among the heterodonor ligands, the strong sigma -donor ligands yielded higher hydroformylation activity than the less basic ligands. In the case of propene, heterodonor bidentate ligands suppressed the hydroformylation reaction. In addition, 1,4-bis(diphenylphosphino)butane and (o-thiomethylphenyl)bis(1-naphthyl)phosphine favoured the formation of n-butanal, Only the -CF3 modified ligands behaved similarly with the two alkenes. The influence of the structure of the ligand in hydroformylation was studied by using molecular modelling methods. The steric size of the ab initio energy minimised free ligand structures was evaluated by Tolman's cone angle method. Co-ordination properties of -CF3 group modified phosphines were studied in reaction with Rh-2(CO)(4)Cl-2, and the structure of Rh(CO)CI(P(o-CF3C6H4)Ph?)2 (1) was determined crystallographically.