The Fenton-like oxidation of the anionic azo-dye Orange II (100-500 mg/L) was batchwise performed using commercial grade diatomites (3.5% Fe content) thermally treated. Solid samples were thoroughly characterized by several techniques. Peroxidation experiments were performed varying the diatomite calcination temperature (500, 700, 1000 degrees C), reaction temperature (50, 60, 70, 80 degrees C), catalyst load (0.47, 0.94, 1.89 and 3.78g), H2O2 concentration (11.0, 13.7, 20.6 mmol/L) and dosing, pH (2-3.5) and initial dye concentration (0.28, 0.57, 1.43 mmol/L). The influence of NaCl and oxalic acid on the catalytic performance and stability was also addressed. The best results were obtained with samples calcined at 700 degrees C, with initial pH 3, at 70 degrees C and using the stoichiometric amount of H2O2, since complete decoloration, TOC reduction close to 67% and negligible Fe leaching were achieved. The stability of the catalyst maintains after 20 h of usage, with a final Fe loss of 2.25%. An average of 0.88 mg/L of iron leached after each run, which is below the discharge limit. (C) 2015 Elsevier B.V. All rights reserved.