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Steroid estrogens are a group of endocrine disrupting compounds (EDCs) excreted in human urine mainly as glucuronide or sulfate conjugates, which might be converted back into the free estrogens in water. The natural estrogen 17 beta-estradiol (E2) and the synthetic one 17 alpha-ethinylestradiol (EE2) are of particular concern due to their strong estrogenic activity. The main environmental source of estrogen exposure is treated wastewaters, with the associated risk of releasing these pollutants to surface waters and even reaching finished drinking waters. In this work, we have developed and validated (at 0.1 and 1 ng L-1 levels) a robust and sensitive method based on Ultra-High Performance Liquid Chromatography (UHPLC) coupled to MS/MS for the reliable identification and quantification of E2 and EE2 in surface waters. Several sulfate conjugates have been also included into the methodology. A pre-concentration step based on off-line solid-phase extraction with Oasis HLB cartridges has been optimized prior to the UHPLC-(ESI) MS/MS determination with a last-generation triple quadrupole analyzer. Two isotopelabelled analytes (E2-d(4) and EE2-d(4)) have been used as surrogate standards to ensure an accurate quantification, achieving very low limits of detection for E2 (0.06 ng L-1) and EE2 (0.02 ng L-1). Several surface waters and influents and effluents from wastewater treatment plants have been analyzed to test the method applicability.