The water-soluble complex Ru(H) 2 (CO)(TPPMS) 3 where TPPMS: (C 6 H 5 ) 2 P(m-C 6 H 4 SO 3 Na) was obtained by phosphine ligand exchange from the corresponding PPh 3 derivative in a water-toluene mixture and characterized by FTIR, UV-Vis, and 1 H NMR and 3 1 P{ 1 H} spectroscopy. The complex was used as a catalyst precursor for the aqueous-biphasic catalytic hydroformylation of several alkenes and alkene mixtures under moderate reaction conditions. The activity trend observed was: 1-hexene > cyclohexene > 2,3-dimethyl-1-butene and the catalyst was shown to be resistant to possible sulphur poisons.