The choice of the optimal method of municipal waste landfill leachate treatment should take into account such factors as: the variability of their composition and quantity over time, requirements for effluent or economic capacity. Due to the variety of compounds found in landfill leachate, it is adviced to use a combination of different treatment processes. Biological methods are effective in the treatment of leachate from young landfills, but generally its efficiency declines significantly with the age of the landfill. Therefore, the preferred are physico-chemical methods, i.e. advanced oxidation, adsorption, membrane methods or a combination of mentioned. The aim of the study was the assessement of the Fenton's reagent utilization to process reverse osmosis concentrate of leachates collected from municipal non-hazardous waste landfill. The research was focused on the influence of pH on the process, and the effectiveness of organic compounds removal, increasing the biodegradability, as well as humic substances transformations were determined. Concentration of humic compounds were measured by optical density at the wavelengths: 254, 265, 280, 320, 465 and 665 nm. In the experiment, the H2O2/Fe2+ 10: 1 ratio was tested at with a dose of H2O2 equal to 3.4 g/l (0.1 M). It has been found that the pH affects the efficiency of COD removal and the increase of the biodegradability of the landfill leachate retentate, and also, the removal of humic substances and its transformation. It was found that at pH 3 the highest removal of COD occurs, with simultaneous increase in biodegradability, and decrese of absorbance caused by humic compounds.