This investigation aimed to refine the operational parameters of the Fenton process, to compare Fenton-treated and non-treated leachate with respect to physicochemical variables and toxicity towards Daphnia magna, and to characterize the sludge. The optimal conditions for the Fenton treatment involved the use of a reagent containing 12 g Fe²⁺ L⁻¹, H₂O₂/Fe²⁺ molar ratio of 9 and pH 2.0, with oxidation, flocculation and sedimentation times of 30, 10 and 15 min, respectively. Under these conditions, the values of the majority of parameters, including chemical oxygen demand (COD), biochemical oxygen demand, dissolved organic carbon, true color (TC), carbohydrates, proteins, phosphorus, total solids, total volatile solids, dissolved volatile solids, Kjeldahl nitrogen, N-NH₃ and iron, were reduced significantly after treatment. However, sludge production rate remained somewhat high (98 kg m⁻³ of treated leachate), although the specific resistance to filtration of the sludge was moderate (12 × 10⁹ cm g⁻¹). While the Fenton process achieved a satisfactory removal of COD (87%) and TC (91%), the treated leachate contained organic compounds that were resistant to oxidation and remained toxic towards D. magna. Hence we conclude that the Fenton process alone is not appropriate for treatment of leachate, because it could negatively affect the ecosystem in receiving water bodies, but it could represent a viable alternative for the pretreatment of landfill leachate. GRAPHICAL ABSTRACT