Degradation of organic dyes is a matter of great environmental concern. Despite many significant efforts and the numerous systems, which have been exploited, the problem is still unsolved due to intrinsically low activity or use of non-convenient chemical reagents. In this work, we describe the use of an oxidative system based on hydrogen peroxide activated by metalloporphyrins as catalysts to carry out the degradation of methyl orange, a model azo dye, in fairly high concentrations. Some inhibition of the catalytic activity during the reaction was originally observed. Adjustments in the system were made in order to prevent this inhibition and improve the efficiency of the system. (c) 2005 Elsevier B.V. All rights reserved.