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Since elevated amounts of chlorine disinfectant were discharged into surface water, more attention should be paid to the reactions between dissolved organic matter (DOM) and chlorine under sunlight. However, disinfection byproducts (DBPs) formed from DOM by solar photolysis of chlorine, and changes of cytotoxicity during this process remain unclear. In this study, it was found that solar photolysis of chlorine significantly promoted the formation of aliphatic chlorinated DBPs and aromatic chlorinated DBPs (including chlorobenzoquinone) by 44.7-109 % and 81.7-121 %, respectively compared with dark chlorination. Unknown total organic chlorine contained in low molecular weight fraction (<1 kD) significantly positively correlated to the cytotoxicity of water samples. Several factors (bicarbonate, dissolved oxygen, pH, nitrate, ammonia, bromide, and iodide) affecting the radical chemistry, and the formation of DBPs under solar photolysis of chlorine were also investigated. Reactive species including HO • , Cl • , O 3 , and reactive nitrogen species (RNS) were responsible for forming different DBPs. Especially O 3 increased the formation of most categories of DBPs tested in this study, and RNS contributed to the formation of nitrogenous DBPs. This study provided more understanding of the adverse impact of overused chlorine, and reaction mechanisms between reactive species and DOM., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)