The hydroformylation of long-chain olefins catalyzed by the water soluble rhodium complex, RhCl (CO) (TPPTS) 2, were studied in aqueous/organic biphasic system containing cationic surfactants. The addition of TPPDS as a modifier dramatically increased the regioselectivity of olefin hydroformylation. A synergistic effect of TPPTS and TPPDS on the regioselectivity was observed. The ratio of linear/branched aldehyde rose from 6.5 (without TPPDS) to 22.3 ([TPPTS]/[TPPDS]=2:1). The steric structure of hydrophilic group in the cationic surfactants exhibited an important influence on the regioselectivity. The hydrophilic group with a small steric volume was favorable for the formation of linear aldehyde. It was found that when the alkyl chain length of the higher olefin and that of the cationic surfactant were comparable (matching size), the regioselectivity for linear aldehyde was outstandingly high.