Nonylphenol is a hydrophobic endocrine disrupting compound, which can inhibit the growth of sewage bacteria in biological processes. This study investigated the degradation of 4-n-nonylphenol (NP) in water by a chemical-free technology of sonophotolysis with emphasis on the impacts of several important parameters, including light intensity, solution pH, two commonly seen inorganic ions (i.e. NO3- and HCO3-), and principally on the examination of degradation mechanisms. It was found that, solution pH could significantly influence both NP degradation efficiency and the synergistic effect of sonophotolytic process, where higher synergistic effect was obtained at more acidic condition. In addition, the presence of NO3- accelerated NP degradation by both acting as a photosensitizer and providing NO2 center dot radicals, while HCO3- had little effect on NP degradation. Identification of intermediates of NP degradation indicated that NP sonophotolysis was mainly initiated by the formation of hydroxy-NP, and a new intermediate dihydroxy-NP was identified for the first time ever in this study. Through thermodynamic analysis, results indicated that both ortho- and meta-hydroxy-NP species can coexist in the solution but the ortho-4-NBZQ (4-nonyl-benzoquinone) is dominant. In addition, the mechanism of ortho-hydroxy-NP formation was suggested by the addition of HO center dot and H-center dot radicals. (C) 2016 Elsevier B.V. All rights reserved.