This works deals with the non-biodegradable azo-dye Orange II degradation using the heterogeneous Fenton-like reaction. Two catalyst series based on transition metals (Fe, Co, Ni) were prepared trying to improve not only the catalytic performance but also to diminish the metal leaching. Thus, metal-doped carbon aerogels and supported catalysts, the latter prepared by classical wet impregnation, were tested. The catalysts were characterized by different techniques: N-2 adsorption, mercury porosimetry, X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). Chemical oxidation of the azo-dye Orange II was carried out in a batch reactor, at atmospheric pressure and 30 degrees C. The influence of the preparation method, the porous characteristics of supports and metal dispersions on the metal leaching and catalytic performance were analyzed. Impregnated catalysts are in general more active than metal-doped carbon aerogels, because metal particles are more accessible to reactants, while the doped-carbon aerogels present a smaller leaching tendency. The behaviour of impregnated catalysts is determined not only by the metal Used but also by the porous texture of the supports. Thus, microporosity strongly favours the Orange II adsorption capacity and tends to reduce the metal leaching, while mesoporosity strongly increases the metal dispersion and consequently, the catalytic performance. Iron is seemingly the most active metal tested, exhibiting simultaneously the lower leaching levels for all of the supports. Although cobalt is an interesting alternative in terms of activity, it exhibits unacceptable leaching degrees. (C) 2008 Elsevier B.V. All rights reserved.