Accelerated bleaching, photobleaching and mineralization of the non-biodegradable azo-dye, Orange II, was observed with oxone in solutions with Co2+-ions. The bleaching rate of Orange II in the dark was found to follow a first-order kinetics with respect to [Co2+] with a rate constant of 20 M-1 s(-1). Fitting of the Orange II photobleaching experimental points in the presence of the Co2+/oxone reagent was carried out and followed the trend known for reactions presenting a chain radical branched mechanism. The photobleaching trace could be fitted by a single mathematical expression with an error <5% with respect to the experimental data. The bleaching trace observed for the Orange II solution in the dark followed zero-order decay kinetics. In a typical run, Orange II (0.20 mM or TOC 30 mg C/l) in the presence of oxone concentrations necessary to catalyze the Orange II mineralization and Co2+ was bleached under visible light within &SIM; 15 s. The Co2+-ion concentrations necessary to catalyze the Orange II mineralization by oxone` was observed to be &SIM;100 times lower than the oxone concentration. A 100% TOC decrease, under visible light irradiation, was attained for an Orange II (0.2 mM) solution in the presence of Co2+-ions (0.06 mM) and oxone (20 mM) within times &SIM;70 min for solutions purged with oxygen. Under visible light irradiation, Orange II mineralization in the presence of O-2 involves the photo-dissociation of reaction intermediates leading to organic peroxides in the second step of the mineralization process. (C) 2004 Elsevier B.V. All rights reserved.