Hydroformylation of propene is the most important hydroformylation process on the industrial scale. Typically, the aim has been the regioselective production of the linear product n-butanal, Recently, interest in selective formation of the branched aldehyde, isobutanal, has increased due to its use in the production of polyols, such as neopentyl glycol (2,2-dimethyl-1,3-propanediol). Study was made of the effect of o-alkyl-substituted triphenylphosphine ligands on the activity and regioselectivity in rhodium-catalysed propene hydroformylation. In addition, the effect of the process conditions on the activity and regioselectivity was investigated by varying the ligand-to-rhodium ratio (0-40), temperature (353-403 K), and deactivation for some of the ligands. The results suggest that o-alkylsubstituted triphenylphosphine ligands enhance the selectivity to isobutanal. However, the activity decreases at the same time. A correlation was found between the P-31-NMR shifts of the ligands and the regioselectivity and activity: as the shift decreases, the regioselectivity to isobutanal increases and the activity decreases. Changes in the process conditions had an effect similar to that in the reaction modified with triphenylphosphine.