New non-C-2-symmetric chiral diphosphites have been synthesized and used as ligands in the rhodium catalyzed enantioselective and regioselective hydroformylation of styrene. A synergistic effect improving both activity and enantioselectivity is observed when a chiral dithiourea is introduced as co-ligand in the Rh-diphosphite catalytic system. (c) 2006 Elsevier B.V. All rights reserved.