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Historical pollution with hexachlorocyclohexane (HCH) isomers of soil and groundwater unfortunately is an unsolved problem, especially in some countries where Lindane is still produced, as also in Europe in the surrounding areas of former production sites or landfilling sites usually due to inadequate long-term storage, treatment or recovery of these wastes. HCH removal technologies are usually dedicated to soil remediation, leachate treatment, water and wastewater treatment. This paper presents the research efforts to develop a treatment technology for gamma HCH removal from water matrices by advanced oxidation processes (AOPs) based on ultrasonication, Fenton and photo Fenton oxidation. Five treatment systems were assessed comparatively: direct ultrasonication, oxidation with hydrogen peroxide, ultrasonication with hydrogen peroxide, Fenton ultrasonication and Fenton ultrasonication followed by photo Fenton oxidation. The energy (25-800 kJ) and amplitude of ultrasonic field, initial concentration of HCH (10 - 89 g/L), hydrogen peroxide (1 - 4000 x stoichiometric dose), iron (Fe II) doses (1-15 mg/L) and UV irradiation time (30-60 min.) were the main experimental parameters evaluated. The ultrasonic frequency was constant - 20 kHz in all experiments. Best treatment performance of 99.9% HCH removal efficiency was achieved after application of a treatment train combining Fenton ultrasonication followed by Fenton UV photolysis at the main experimental parameters of: 200 kJ ultrasonic energy, 1000 x s peroxide dose, 5 mg Fe(II)/L and 30 minutes of UV irradiation. [ABSTRACT FROM AUTHOR]