The kinetics of the reaction of HOBt with [Rh(H₂O)₅(OH)]²⁺ has been studied spectrophotometrically in aqueous medium as a function of [Rh(H₂O)₅OH²⁺], [HOBt], pH and temperature. At pH 4.3, the reaction proceeds via a rapid outer sphere association complex formation step followed by two consecutive steps. The first of these involves ligand-assisted anation, while the second involves chelation as the second aqua ligand is displaced. The association equilibrium constant for the outer sphere complex formation has been evaluated together with the rate constants for the two subsequent steps. The activation parameters for both steps have been evaluated using Eyrings equation. Thermodynamic parameters calculated from the temperature dependence of the outer sphere association equilibrium constants are also consistent with an associative mode of activation. The product of the reaction has been characterized by IR and ESI-mass spectroscopic analysis.