The degradation of sulfonamides (SAs) in the presence of bromide during the UV/free chlorine process was investigated in this study. The reactions followed pseudo first-order kinetics. With increasing bromide concentrations of 1, 5, 10-20 mu M, the degradation rate was gradually raised. The contribution of different reactive species has been calculated. The degradation products and DBPs formed by SAs were determined. The mechanisms of the degradation of SAs in the presence of bromide included hydroxylation, cleavage of the pyrimidinyl group, and bromine and chlorine electrophilic addition. The concentration of brominated disinfection by-products, including DBCM, TBM and DBAN, increased with increasing bromide dosage and degradation time, while 1, 1, 1-TCP and 1, 1-DCP decreased. In an acute toxicity test, the inhibition rate decreased in the first 5-10 min then increased after 10 min degradation when 10 mu M Br- was added to the system. The initial decrease may due to the formation of low toxicity substances after the cleavage of the pyrimidinyl group, while the subsequent increase may be attributable to the increased formation of DBPs.