Effects of bromide (Br-) and iodide (I-) on chlorination kinetics of diclofenac (DCF) and potential disinfection by-products (DBPs) formation were investigated in this study. The presence of either Br- or I- accelerated the chlorination of DCF with different extents. Then species-specific apparent second-order rate constants (k(app)) of DCF with free chlorine (FC), free bromine (FB), free iodine (FI) were determined. It was found that the reactivity rate of FB with DCF was higher than those of FC and FL The k(app,FB-DCF) at pH 7.0 was determined to be 9320 M-1 s(-1), which was 3680 and 3.2 folds of those of FC and FI, respectively. The k(app,FC-DCF) were highly pH-dependent, while the k(app) values of DCF with FB and FI were less influenced by pH than FC. The presence of Br- and I- enhanced the formation amounts and altered the speciation of DBPs. The DBPs formation amounts were enhanced from 67.2 to 292.0 mu g L-1 and 87.3 mu g L-1, respectively, when 50 mu M Br- or I- was spiked. Meanwhile, the dominant species of formed DBPs shifted from chlorinated DBPs to brominated and iodinated DBPs. At last, the transformation by-products and pathways of DCF under chlorination, chlorination with Brand chlorination with I- were proposed and compared. The DCF transformation pathways mainly proceeded through hydroxylation, decarboxylation and substitution reactions. Although the presence of Br- and I- accelerated the chlorination of DCF, the enhanced formation of brominated and iodinated DBPs highlights the necessity to remove Br- and I- from source water prior to chlorination for mitigating the formation of toxic DBPs.