Aqueous ( D2O) solutions of tetrakis( 3,5- disulfonatomesityl) porphyrin rhodium( III) aquo/ hydroxo complexes ([( TMPS) Rh-III(D2O)(2)](-7) ( 1), [( TMPS) Rh-III(OD)( D2O)](-8) ( 2), and [( TMPS) Rh-III(OD)(2)](-9) ( 3)) react with hydrogen ( D-2) to form an equilibrium distribution with a rhodium hydride ([( TMPS) Rh - D( D2O)]- 8 ( 4)) and a rhodium(1) complex ([(TMPS) Rh1( D2O)](-9) ( 5)). Equilibrium constants (298 K) are measured that define the distribution for all five of these ( TMPS) Rh species in this system as a function of the dihydrogen (D-2) and hydrogen ion (D+) concentrations. The hydride complex [( TMPS) Rh - D( D2O)](-8) is a weak acid in D2O ( K a(298 K)) 4.3 x 10(-8)). Steric demands of the TMPS porphyrin ligand prohibit formation of a Rh( II) - Rh( II)- bonded complex, related rhodium( I) - rhodium( III) adducts, and intermolecular association of alkyl complexes which are prominent features of the rhodium tetra( psulfonatophenyl) porphyrin (( TSPP) Rh) system. The rhodium( II) complex ([( TMPS) Rh-II( D2O)]- 8) reacts with water to form hydride and hydroxide complexes and is not observed in D2O. The ( TMPS) Rh - OD and ( TMPS) Rh - D bond dissociation free energies ( BDFE) are virtually equal and have a value of approximately 60 kcal mol(-1). Reactions of [( TMPS) Rh - D( D2O)]- 8 in water with CO and olefins produce rhodium formyl and alkyl complexes which have equilibrium thermodynamic values comparable to the values for the corresponding substrate reactions of [( TSPP)Rh - D( D2O)](-4).