Various new chiral thioureas have been synthesized and used as ligands for the asymmetric hydroformylation of styrene catalyzed by rhodium(I) complexes. The best results were obtained with N-phenyl-N'-(S)-(1-phenylethyl)-thiourea associated to a cationic rhodium(I) precursor, and asymmetric induction of 40% was then achieved. As enantioselectivity is obtained with low conversions, various parameters have been examined in order to increase the catalytic activity without the loss of asymmetric induction.