Isotopic exchange between D-2 (g) and H2O (1) is catalyzed by the water soluble complexes Rh(TPPTS)(3)Cl and Rh(TPPMS)(3)Cl (where TPPTS = tris(3-sulfonatophenyl)phosphine, trisodium salt and TPPMS = 3-sulfonatophenyldiphenylphosphine, sodium salt). This isotope exchange has been monitored by gas chromatographic analysis of the gaseous headspace over the aqueous catalyst solution. Upon binding to the catalyst, the hydrogen isotopologues can either exchange one or both atoms with solvent. Kinetic schemes have been developed for both situations. Analysis of exchange data demonstrates that for both catalysts at pH >= 8 the D-2 Molecule only undergoes one exchange event per visit to the active site. At lower pH values this changes. Rh(TPPMS)(3)Cl shows evidence of double exchange at pH 7 and below. Rh(TPPTS)(3)Cl shows evidence of double exchange at pH 6 and below. Rate constants for both single and double exchange can be determined and have been analyzed in the pH range from 3 to 12. Mechanistic implications of the rate constants are discussed. (c) 2006 Elsevier B.V. All rights reserved.