In this research, the brominated oligomers formed during phenol degradation with boron-doped diamond (BDD) anode had been initially studied at three different concentrations of bromide (1, 10 and 100 mM). The results from LC/MS analysis indicated that, brominated monomer, dimer and trimer of phenols resulting from electrophilic substitution and coupling reactions were the important reaction by-products. Specifically, the trimer by-products were generated only in bromide-rich systems. The reaction mechanisms concerning oligomer formations were proposed in detail accordingly. The above results were in well accordance with those recorded in the degradation experiments. As a whole, bromides and chlorides demonstrated quite different effects toward phenol degradation, which deepened our understanding on the reactions involved in BDD anode cells. (C) 2017 Elsevier B.V. All rights reserved.