A visible spectrophotometric method has been developed for the reaction kinetics of o-phenylenediamine in the presence of gold (III). The method is based on the measurement of the absorbance of the reaction o-phenylenediamine and gold (III). Optimum conditions for the reaction were established as pH 6 at lambda=466 nm. When the reaction kinetic of o-phenylenediamine by gold (III) was investigated, it was observed that the following rate formula was found as In (A/A(0)) = -kt, according to absorbance measurements. The activation energy E-a and Arrhenius constant A were calculated from the Arrhenius equation as 1.009 kJ center dot mol(-1) and 3.46 center dot 10(-2) s(-1), respectively. Other activation thermodynamic parameters, entropy, Delta S (J center dot mol(-1) center dot K-1), enthalpy, Delta H (kJ center dot mol(-1)), Gibbs free energy, Delta G (kJ center dot mol(-1)) and equilibrium constant, K-e were calculated at T = (283.2, 303.2, 323.2, and 343.2) K. The study was exothermic due to the decrease of entropy and was a non-spontaneous process during activation. (c) 2007 Published by Elsevier Ltd.