There has been a series of research work on exploring how disinfection by-products (DBPs) are formed from free amino acids (AAs) during chlorination in drinking water. Yet there is very limited information about the formation of carbonaceous DBPs (C-DBPs) and nitrogenous DBPs (N-DBPs) from the chlorination of those AAs in combined forms which are more abundant than free AAs in source waters. In this study, we compared the differences on the formation of trihalomethanes (THMs, representing C-DBPs) and haloacetonitriles (HANs, representing N-DBPs) from free AAs and combined AAs (short oligopeptides) during chlorination. And, the influence of the selected pre-oxidation processes (permanganate, persulfate, hydrogen peroxide) on THM and HAN formation during subsequent chlorination was also examined. There were significant differences in chloroform (CF), dichloroacetonitrile (DCAN) and trichloroacetonitrile (TCAN) formation between free AAs and short oligopeptides, which was probably ascribed to the presence of peptide bond. Of note, the combined tyrosine (Tyr) in tripeptides (Tyr-Tyr-Tyr) formed more TCAN compared with free Tyr and other combined Tyr in dipeptides, and a novel possible formation pathway of TCAN from Tyr-Tyr-Tyr was proposed. In addition, the effect of three pre-oxidation on DBP formation was different, and their rank order was permanganate > persulfate > hydrogen peroxide. The permanganate is a better choice than persulfate and hydrogen peroxide for controlling AA derived HANs, but this benefit should be weighed against the potential drawback of increased regulated THMs. (C) 2015 Elsevier B.V. All rights reserved.