Degradation of sodium alkylbenzenesulfonate (ABS) in the Fenton reagent in the presence of Cl- ions has been studied. An anionic surfactant sodium alkylbenzenesulfonate (C₁₁-C₁₄) has been selected as a model compound. The effect of individual components of the reactive system (H₂O₂, H₂O₂/Cl-, Fe²⁺/H₂O₂, Fe²⁺/H₂O₂/Cl-) on the degree of ABS degradation has been determined. The degree of ABS degradation has been found to be affected by the presence of Cl- ions and the mutual molar ratios of individual components of the system studied. Moreover, it influences on the composition of the post-reaction mixture. The use of the Fenton reagent in the presence of Cl- ions resulted in the formation of chloroorganic compounds in the solution studied. 2,4,6-trichlorophenol was identified in reaction products and its presence was confirmed using the HPLC - MS method and by HPLC with an internal standard. No chloroproducts were identified during degradation of ABS by the system H₂O₂/Cl-. When only H₂O₂ was applied, the concentration of the surfactant was unchanged.