Humic acid (HA), fulvic acid (FA) and hydrophilic fraction of fulvic acid (FAHPI) were isolated from bottom sediments of a drinking water reservoir in North China. The effects of ultraviolet (UV) irradiation on the structure of the sedimentary natural organic matter (NOM) and subsequent disinfection by-products (DBPs) formation were investigated, with the comparison of commercial humic substances. HA sample mainly consisted of aromatic, methyl and guaiacyl lignin groups, which were reactive with chlorine to form trichloromethane (TCM). FA and FAHPI samples mainly contained ester, phenolic hydroxyl and aromatic ketone groups, among them, phenolic hydroxyl groups were highly reactive toward chlorine and primarily formed dichloroacetic acid (DCAA) during chlorination. UV irradiation decreased the hydrophobicity and increased the hydrophilicity of NOM. UV irradiation eliminated the aromatic, methyl and guaiacyl lignin groups of the HA sample, decreasing the formation of TCM, but increased the phenolic hydroxyl groups of FA and FAHPI samples, enhancing the formation of DCAA. In addition, UV irradiation promoted the trichloroacetic acid (TCAA) formation of the NOM samples from subsequent chlorination. Medium pressure (MP) UV irradiation had greater impacts on NOM fractions and DBPs formation during subsequent chlorination. (C) 2015 Elsevier B.V. All rights reserved.