The concerns over 17α-ethynylestradiol (EE2) have been growing in the recent years due to its increasing presence in the environment and its high estrogenic potency. However, there is a limited amount of research available on the photochemical behaviors of EE2 in the presence of various aquatic environmental matrix components even though photodegradation has been considered an effective pathway for the removal of organic pollutants from not only waste waters but also natural aquatic ecosystems. Herein, the direct photodegradation of EE2 and the effects of common natural water matrix components, humic acid and carbonate ions, on the photodegradation were studied. High-Performance Liquid Chromatography (HPLC) was utilized to determine the concentrations of EE2. The direct photodegradation of EE2 was found to increase with increasing water pH value in the range of 7.0 to 10.0 and the photodegradation rate constant increased with increasing concentration of EE2. Humic acid enhanced the photodegaradation rate of EE2 under the simulated sunlight employed, while carbonate (CO 3 2-/HCO 3 −) had shown slight influences on EE2 photodegradation.