Hydrolysis and hydrogenation of [RhCl(tppms)(3)] (1) and trans-[RhCl(CO)(tppms(2)] (2) was studied in aqueous solutions in a wide pH range (2 < pH < 11) in the presence of excess TPPMS (3 -diphenylphosphinyl-benzenesulfonic acid sodium salt). In acidic solutions hydrogenation of 1 yields a mixture of cis-mer- and cis;fac-[RhCIH2(tppms)(3)] (3a,b) while in strongly basic solutions [RhH(H2O)(tppms)(3)] (4) is obtained, the midpoint of the equilibrium between these hydride species being at pH 8.2. The paper gives the first successful H-1 and P-31 NMR spectroscopic characterization of a water soluble rhodium(I)-monohydride (4) bearing only monodentate phosphine ligands. Hydrolysis of 2 is negligible below pH 9 and its hydrogenation results in formation of [Rh(CO)H(tppms)(3)] (5), which is an analogue to the well known and industrially used hydroformylation catalyst [Rh(CO)H(tppts)(3)] (6) (TPPTS = 3,3',3"-phosphinetriyltris(benzenesul-fonic acid) trisodium salt). It was shown by pH-potentiometric measurements that formation of 5 is strongly pH dependent in the PH 5-9 range; this gives an explanation for the observed but previously unexplained pH dependence of several hydroformylation reactions. Conversely, the effect of pH on the rate of hydrogenation of maleic and fumaric acid catalyzed by 1 in the 2< pH < 7 range can be adequately described by considering solely the changes in the ionization state of these substrates. All these results warrant the use of buffered (pH-controlled) solutions for aqueous organometallic catalysis.