The effect of pH on the formation and equilibrium distribution of the water soluble ruthenium hydrides [HRuCl(TPPMS)(2)](2), [HRuCl(TPPMS)(3)] and [H2Ru(TPPMS)(4)] (TPPMS=(3-sulfonatophenyl)diphenylphosphine sodium salt) was studied in aqueous solution by pH-potentiometric and H-1 and P-31 NMR methods. Depending on the pH, [RuCl2(TPPMS)(2)](2) and its hydride-derivatives hydrolyse extensively, giving rise to formation of hydroxo-ruthenium complexes. It was established that at pH less than or equal to 3.3 the dominant ruthenium(II) species was [HRuCl(TPPMS)(3)], while at pH greater than or equal to 7 it was [H2Ru(TPPMS)(4)]. While [HRuCl(TPPMS)(3)] catalyzed the slow, selective hydrogenation of the C=C bond in trans-cinnamaldehyde, [H2Ru(TPPMS)(4)] was found an active and selective catalyst for C=O reduction. Consequently, the selectivity of the hydrogenation of trans-cinnamaldehyde could be completely inverted by minor changes in the solution pH, shifting the equilibrium between [HRuCl(TPPMS)(3)] and [H2Ru(TPPMS)(4)].