Ultraviolet/persulfate (UV/PS) has been widely used to generate sulfate radicals (SO center dot(-)(4)) in degrading organic pollutants. However, its impact on chlorinated disinfection byproducts (DBPs) is unclear. Impact of UV/PS pretreatment on the formation of DBPs during subsequent chlorination of natural organic matter was evaluated in this study. For the samples pretreated by UV/PS, the formation of some carbonaceous disinfection byproducts (C-DBPs) such as chloroform and haloacetic acids only increased a little, but the formation of nitrogenous disinfection byproducts (N-DBPs) including haloacetonitriles and trichloronitromethane decreased slightly. UV/H2O2 pretreatment increased the formation of both C-DBPs and NDBPs significantly with the initial dosage of H2O2 being 30 mu M. Chloral hydrate and haloketones followed an increasing and then decreasing pattern, but dichloroacetonitrile and trichloronitromethane followed an opposite pattern with increasing UV/PS pretreatment time. Most of the C-DBPs rose monotonically with increasing dosages of PS, but N-DBPs declined at the dosage of PS being 10 mu M by UV/PS pretreatment. Preoxidation pH and the presence of nitrate played little role in DBP formation. In the presence of 2.5 mg/L ammonia, UV/PS pretreatment increased most of the DBPs except for trichloroacetonitrile and trichloronitromethane. Bromine incorporation factors for trihalomethanes, haloacetic acids and dihaloacetonitriles were affected by UV/PS preoxidation insignificantly. It is suggested that the combination of UV/PS and post chlorination was expected to be a good choice in controlling the formation of DBPs. (C) 2015 Elsevier B.V. All rights reserved.