Photodegradation is an important abiotic pathway transforming organic pollutants in natural waters. Humic substances (HS), including humic and fulvic acids, are capable of accelerating the photodegradation of steroid estrogens. However, how the photodegradtion of the emerging pollutants influenced by HS is not clear. Thus, we studied the roles and mechanisms of HS in inducing the photodegradation of 17 alpha-ethynylestradiol (EE2). HS generally induces EE2 photodegradation through binding and reactive species generation. Apart from hydroxyl radical (HO center dot), the excited triplets of humic substances ((HS)-H-3*) are other key reactive species degrading EE2 by abstracting electrons. HO center dot and (HS)-H-3* were responsible for about 60% of the overall EE2 photodegradation at 250 mu mol HS L-1. Most of EE2 molecules bound to the HS via H-bonding, pi-pi and hydrophobic interactions. The binding role of HS in promoting EE2 photodegradation was rationalized by 17 beta-estradiol competitive binding with EE2 to the humic and fulvic acids. Furthermore, HS-promoted photodegradation can alter EE2 toxicity to wheat, rice and Ormosia plants. This study extends our knowledge on the photochemical behaviors and ecological risks of steroid estrogens in natural waters. (C) 2016 Elsevier B.V. All rights reserved.