Please cite this article in press as: Z. Frontistis, et al., UV and simulated solar photodegradation of 17␣-ethynylestradiol in secondarytreated wastewater by hydrogen peroxide or iron addition, Catal. Today (2014), http://dx. a b s t r a c tThe extensive use of estrogens and their release, through various pathways, into the environment, constitutes an emerging environmental problem that poses serious threats onto public health. In this work the efficiency of UVC/H 2 O 2 and solar/Fe 2+ treatment to degrade 17␣-ethynylestradiol (EE2) in environmentally relevant concentrations of 100 g/L in secondary-treated wastewater matrices was investigated. Also, photolytic treatment was performed under different irradiation sources, namely UVC, UVA and simulated solar light. The effect of H 2 O 2 (0-20 mg/L) and Fe 2+ (0-15 mg/L) concentration was investigated and, at optimal operating parameters, EE2 removal was 100% after 15 min of UVC/H 2 O 2 treatment, while EE2 removal reached 86% after 60 min of solar/Fe 2+ treatment. In addition, the effect of water matrix and pH was studied. Total organic carbon (TOC) and yeast estrogen screening (YES) measurements showed the formation of stable intermediate products during EE2 treatment and an attempt to elucidate the reaction pathways and mechanisms through the identification of transformation products (TPs) by means of UPLC-MS/MS was made. Several TPs, including quinone methide and 1,2-quinone derivatives, were identified and competing pathways were suggested, in which hydroxylation, alkylation, dealkylation, demethylation and dehydroxylation, amongst others were described as major transformation mechanisms.