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17α-Ethynylestradiol (EE2) in natural waters may cause adverse effects on organisms due to its high estrogenic potency.Laboratory studies were performed to study the effects of a local humic acid (LHA),fulvic acid (LFA) and Aldrich humic acid (AHA) on the photochemical behavior and estrogenic potency of EE2.Here photolytic experiments demonstrated that pure aqueous EE2 could undergo direct and self-sensitized photodegradation at a global rate of 0.0068 hr-1.Photodegradation rate of EE2 in 5.0 mg/L.dissolved humic substances (DHS) was determined to be 0.0274,0.0296 and 0.0254 hr-1 for LHA,LFA and AHA,respectively.Reactive oxygen species (ROS) and triplet dissolved humic substances (3DHS*) scavenging experiments indicated that the promotion effect of DHS on EE2 photodegradation was mainly aroused by the reactions of HO· (35％-50％),1O2 (＜ 10％) and 3DHS* (22％-34％).However,the photodegradation of EE2 could also be inhibited when DHS exceeded the threshold of 10 mg/L.Three hydroxylation products of EE2 were identified using GC-MS and their formation pathways were also proposed.In vitro estrogenicity tests showed that EE2 was transformed into chemicals without estrogenic potency.These findings could extend our knowledge on the photochemical behaviors of steroid estrogens in sunlit natural waters.