Thermodynamic and kinetic parameters for the oxidative addition of H-2 to [Rh-I(bpy)(2)](+) (bpy = 2,2'-bipyridine) to form [Rh-III(H)(2)(bpy)(2)](+) were determined from either the UV-vis spectrum of equilibrium mixtures of [Rh-I(bpy)(2)](+) and [Rh-III(H)(2)(bpy)(2)](+) or from the observed rates of dihydride formation following visible-light irradiation of solutions containing [Rh-III(H)(2)(bpy)(2)](+) as a function of H-2 concentration, temperature, and pressure in acetone and methanol. The activation enthalpy and entropy in methanol are 10.0 kcal mol(-1) and -18 cal mol(-1) K-1, respectively. The reaction enthalpy and entropy are -10.3 kcal mol(-1) and -19 cal mol(-1) K-1, respectively. Similar values were obtained in acetone. Surprisingly, the volumes of activation for dihydride formation (-15 and -16 cm(3) mol(-1) in methanol and acetone, respectively) are very close to the overall reaction volumes (-15 cm(3) mol(-1) in both solvents). Thus, the volumes of activation for the reverse reaction, elimination of dihydrogen from the dihydrido complex, are approximately zero. B3LYP hybrid DFT calculations of the transition-state complex in methanol and similar MP2 calculations in the gas phase suggest that the dihydrogen has a short H-H bond (0.823 and 0.810 angstrom, respectively) and forms only a weak Rh-H bond (1.866 and 1.915 angstrom, respectively). Equal partial molar volumes of the dihydrogenrhodium(I) transition state and dihydridorhodium(III) can account for the experimental volume profile found for the overall process.