Photochemical transformation of steroid estrogens induced by dissolved organic matter has attracted increasing interest due to their extensive presence in natural waters. However, comparative understanding of their photodegradation in aquatic humic acid (HA) and fulvic acid (FA) solutions with different water matrixes is still limited. Herein, 17 alpha-ethinylestradiol (EE2) was selected as a model compound, and photodegraded in riverine water HA and FA solutions as a function of water matrixes to examine the differences in its degradation kinetics and mechanisms. Compared to EE2 photodegradation in HA solution, it was photochemically transformed at a higher rate in FA solution. Quenching experiments revealed that hydroxyl radical was identified as the primary contributor to EE2 degradation in HA solution, while it was a secondary factor in FA solution compared to triplet-state excited species. Due to differential in the predominant mechanisms, EE2 photodegradation in FA solution was promoted in oxygen-poor atmospheres, while an opposite effect was observed for EE2 in HA solutions. The influence of different water matrixes on EE2 photodegradation was clearly different. pH could alter the direct and indirect photolysis of EE2 concurrently, resulting in a constant contribution of the hydroxyl radical, singlet oxygen and triplet-state excited species to EE2 photodegradation. Bicarbonate and chloride ion exhibited no influence on the induced EE2 photodegradation by the HA and FA, while nitrate worked synergistically with HA and FA in accelerating EE2 photodegradation. These findings are helpful for better understanding the photochemical behavior and fate of EE2 in diverse aquatic systems. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.