The kinetics of the catalyzed oxidative mineralization of the thionine dye (TH) (3,7-diamino-phenothiazin-5-ylium acetate) with H2O2 has been studied. The catalysts used were supported silica, alumina, and cation-exchange resins (Dowex-50W, 2 and 8% DVB) in the form of transition metal complexes. These complexes were [Cu(amm)(4)](2+), [Co(amm)(6)](2+), [Ni(amM)(6)](2+), [Cu(en)(2)](2+), and [Cu(ma)(4)](2+) (amm = ammonia, en = ethylenediamine, and ma = methylamine). The reaction exhibited first-order kinetics with respect to thionine dye concentration, [TH], while the order with respect to [H2O2] was dependent on the catalyst type and the concentration range. The reactivity of the catalysts is correlated with the redox potential of metal ions, the amount of complex per gram of air-dried support, and the type of support. The rate of the reaction increased with increasing pH of the medium. Addition of P-cyclodextrin exhibited three distinct effects depending on the concentration range. The inclusion of the substrate and the oxidizing species has been evaluated. The irradiation of the reaction with UV-Iight enhanced the rate of TH mineralization by about 20%. The reaction was enthalpy-controlled as confirmed from the isokinetic relationship. A reaction mechanism was proposed with the formation of free radicals as reactive intermediates. (C) 2003 Society of Chemical Industry.