Catalytic wet peroxide oxidation (CWPO) with an Al/Fe-pillared clay catalyst was assessed in the treatment of stabilized leachate of landfill (COD = 5000-7000 mg O(2)/dm(3); BOD(5) similar to 800 mg O(2)/dm(3); pH = 8.0; BOD(5)/COD= 0.11-0.16). The effects of catalyst loading (0.5-2.0 g(catalyst)/100 cm(leachate)(3)), added peroxide concentration (2.34-4.68 mol/dm(3)) and its rate of stepwise addition (7.5-30 cm(3)/h) in a semi-batch reactor, were studied with COD removal and biodegradability index (BI)(1) as response parameters. COD removals up to 50% and BI output values exceeding 0.3 were achieved in 4 h of reaction at very mild conditions of room temperature and atmospheric pressure (291 K; 72 kPa). High catalyst loadings together with low peroxide concentrations and dosages (below stoichiometric for total mineralization) and low peroxide addition rates were found to provide better use of the oxidizing agent in terms of both, BI enhancement and COD removal. The clay catalyst resulted quite stable against the chemical leaching of the active iron along the catalytic reactions. (C) 2011 Elsevier B.V. All rights reserved.