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Dissolved organic matter (DOM) have been reported as precursors of disinfection byproducts (DBPs) and its molecular characteristics are rarely investigated due to its complexity. In this study, changes in the characteristics of DOM were investigated in the lower Phong River in Thailand in dry season and after the first rain in rainy season, using a non-targeted analysis with Orbitrap mass spectrometry. The river was rich with CHO features dominated by lignin-like molecules, while lipid-like molecules increase after domestic wastewater discharges. Wastewater discharge released DOM with higher molecular weight (MW) that was less oxygenated (low O/C) and less oxidized (low carbon oxidation state [C os ]). A lake affected by anthropogenic activities contributed more oxidized DOM into the river, while surface runoff carried DOM that is more oxygenated (high O/C), less hydrogenated (low H/C), and more oxidized (high C os) to the stream. Water treatment further modified DOM to be lower MW. Approximately three hundred Cl-containing features (CHOCl) detected upstream were also found downstream. Disinfection by chlorine (Cl 2) or chlorine dioxide (ClO 2) formed both CHO and CHOCl DBPs. Low chlorine dosage applied to upstream and downstream samples resulted in many common unknown DBPs while increasing chlorine dosage resulted in more unique DBPs. At the same dosage, Cl 2 reacted with DOM more than ClO 2 , including more oxidized molecules that are refractory to ClO 2. Both Cl 2 and ClO 2 produced chlorinated and non-chlorinated DBPs, and some DBPs were commonly found by both disinfections. Cl 2 -produced DBPs were more unsaturated (higher [DBE–O]/C) and oxidized (higher C os) than ClO 2 -DBPs. Image 1 • Molecular changes in dissolved organic matter (DOM) along the river were tracked. • Domestic discharges release DOM with high molecular weight (MW) and less oxidized. • A natural lake contributed more oxidized DOM into the river. • Water treatment further modified DOM to be lower MW. • Unique and shared CHO and CHOCl disinfection byproducts (DBPs) were found by chlorine or chlorine dioxide. [ABSTRACT FROM AUTHOR]