Mature leachates are a significant cause of soil and water contamination because they contain high organic loads of recalcitrant materials. The application of a physicochemical process is normally insufficient to remove the organic load from these liquids. The objective of the study was to evaluate removal efficiencies in terms of Chemical Oxygen Demand in mature leachates from the Tuxtla Gutierrez landfill applying a physicochemical Coagulation-Flocculation-Fenton system. In the first stage, ferric chloride (FeCl₃) and ferrous sulfate (FeSO₄) coagulants were used at different doses (2.2, 2.6 and 2.8 g L⁻¹) and pH values (4, 5 and 6) in a 2 x 3 x 3 experimental arrangement. This effluent was treated in a second stage (Fenton), testing several mass ratios, ranging from 1 to 3, of the oxidant to the catalyst (H₂O₂/Fe²⁺), maintaining the constant catalyst dose (0.434 g L⁻¹ Fe²⁺). The tests were performed under a 3² factorial design at various pH values (2.5, 3.0 and 3.5) and H₂O₂ doses (300, 550 and 800 mg L⁻¹). The oxidation and flocculation stages were performed at 135 rpm for 80 min and at 20 rpm for 20 min, respectively. The highest COD removal (66%) in the coagulation-flocculation process was obtained with 2.2 g L⁻¹ of FeCl₃ at pH = 6. With the best treatment Fenton reached 70% COD removal at pH 2.5 and 800 mg L⁻¹ of H₂O₂. The tests under this physicochemical system are the first application to recalcitrant leachates in Mexico, reaching a 90% overall efficiency and improving the biodegradability index by 64% from 0.14 to 0.23.