Chloro- and hydrido-complexes of ruthenium(II) with potassium (4-diphenylphosphino)benzenesulfonate (pTPPMS) were prepared and their properties were compared with those of the related complexes of (3-diphenylphosphino)benzenesulfonates. An X-ray crystal structure determination revealed that the para-sulfonated ligand has a Tolman cone angle slightly smaller than that of PPh3. It was established by pH-potentiometric measurements and by H-1 and P-31 NMR studies that the distribution of the hydride derivatives [RuClH(pTPPMS)(3)] and [RuH2(pTPPMS)(3,4)] is strongly pH-dependent. Hydrogenation of trans-cinnamaldehyde in aqueous-organic mixtures is catalyzed by [RuCl2(pTPPMS)(4)] and is also strongly influenced by the pH of the aqueous phase due to the pH-dependent formation of the above hydrides. In acidic solutions (pH < 2) an exclusive C=C reduction takes place, while in alkaline solutions (pH > 6) a selective C=O hydrogenation was observed.