Cylindrospermopsin (CYN) has become a significant environmental concern because of its extremely high toxicity and widespread distribution. TiO2 assisted photocatalytic degradation process has been proven to be effective for CYN degradation. However, the lack of investigation on byproducts and pathways of CYN photocatalytic destruction limited the assessment of this treatment technology (e.g. toxicity of effluent). Therefore, the reaction byproducts and pathways of CYN degradation by TiO2 photocatalysis were studied in this work by using high-performance liquid chromatography combined with quadrupole time-of-flight electrospray ionization tandem mass spectrometry (LC/Q-TOF-ESI-MS). The reaction pathways were proposed and discussed based on the detected byproducts and hydroxyl radical chemistry. The byproducts m/z 432.12 and 448.11 can be formed through hydroxylation by (OH)-O-center dot hydrogen abstraction and addition to double bonds. Byproducts without the sulfate group can be generated through a sulfate elimination process. Byproducts (such as m/z 392.12 and 375.10) formed after ring opening by attacks on the hydroxymethyl uracil moiety and tricyclic guanidine moiety were also identified in this study. The detoxification study showed an effective removal of cytotoxicity after TiO2 photocatalytic treatment. This work is important in the proper assessment of the photocatalytic destruction of CYN in water and provides insights that can help determine the level of treatment required by such advanced oxidation technologies. (C) 2014 Elsevier B.V. All rights reserved.