Accelerated bleaching, photobleaching and mineralization of the non-biodegradable azo-dye, Orange II, was obsd. with oxone in solns. 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 const. of 20 M-1 s-1. Fitting of the Orange II photobleaching exptl. 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 math. expression with an error <5% with respect to the exptl. data. The bleaching trace obsd. for the Orange II soln. 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 and Co2+ was bleached under visible light within .apprx.15 s. The Co2+-ion concns. necessary to catalyze the Orange II mineralization by oxone was obsd. to be .apprx.100 times lower than the oxone concn. A 100% TOC decrease, under visible light irradn., was attained for an Orange II (0.2 mM) soln. in the presence of Co2+-ions (0.06 mM) and oxone (20 mM) within times .apprx.70 min for solns. purged with oxygen. Under visible light irradn., Orange II mineralization in the presence of O2 involves the photo-dissocn. of reaction intermediates leading to org. peroxides in the second step of the mineralization process. [on SciFinder (R)]