This study reports the kinetically fast and efficient decoloration/degradation of solutions of the azo dye Orange II photocatalyzed by a Nafion-Fe(1.78%) membrane in a photoreactor. Orange II solutions (30 ppm C), decolored under 36-W light in the presence of H2O2 (5.0 mM) in less than 0.5 h. The decoloration process was optimized via statistical modeling to attain the most economic. use of energy, chemicals and time. Experimental evidence is presented for the dye sensitizing the degradation process. Orange II degradation is controlled by mass transfer and not by the chemical reaction of the oxidative radicals in solution. It was possible to attain a BOD5/COD ratio of similar to0.4 during the reactor pretreatment. A steep increase in the value of BOD5 was observed for pretreated solutions of Orange II because of the mal e readily biodegradable intermediates produced in solution.