Abstract This study investigated the effects of preozonation on disinfection by-products (DBPs) formation during chlorination and chloramination of the water collected from Tai Lake. Results showed that the high ozone dose (0.6–1.0mg O3/mg DOC) pretreatment reduced the yields of trihaloacetic acids (reduced 62–63% in chlorination), dihaloacetonitriles (reduced 53–55% and 14–26% in chlorination and chloramination, respectively) and trihaomethanes (reduced 19% in chloramination), but markedly increased the formation of halonitromethanes (increased 4.7–5.6 times in chlorination and 2.1–2.7 times in chloramination), haloketones (increased 4.8–7.1 times in chlorination and 2.5–2.9 times in chloramination) and dihaloacetic acids (increased 1.5–2.4 times in chlorination and 0.3–0.6 times in chloramination). Thus the high ozone dose pretreatment should be avoided during chlorination/chloramination of Tai Lake water. Also, chloramination (with and without preozonation) produced much lower DBPs yields as compared with chlorination (with and without preozonation), indicating that chloramine was a better choice to control the DBPs yields. Further analysis also revealed that the bromine substitution factors (BSFs) of DBPs varied with disinfection mode. In chlorinamination, the BSFs generally showed a decrease trend with the ozone dose, yet in chlorination, the BSFs mostly exhibited first an increase and then a decrease trend. Moreover, the BSFs of DBPs in chloramination (with or without preozonation) were dominantly lower than those in chlorination (with or without preozonation).