The changes in dissolved organic matter (DOM) throughout the treatment processes in a drinking water treatment plant in Sweden and the formation of disinfection by-products (DBPs) were evaluated by using ultrahigh resolution mass spectrometry (resolution ~ 500,000 at m/z 400) and nuclear magnetic resonance (NMR). Mass spectrometric results revealed that flocculation induced substantial changes in the DOM and caused quantitative removal of DOM constituents that usually are associated with DBP formation. While half of the chromophoric DOM (CDOM) was removed by flocculation, about 4-5 mg L-1 total organic carbon remained in the finished water. A conservative approach revealed the formation of about 800 mass spectrometry ions with unambiguous molecular formula assignments that contained at least one halogen atom. These molecules likely represented new DBPs, which could not be prevented by the flocculation process. The most abundant m/z peaks, associated with formed DBPs, could be assigned to C5HO3Cl3, C5HO3Cl2Br and C5HO3ClBr2 by using isotope simulation patterns with the likely DBPs were produced and suggested the presence of halogenated polyphenolic and aromatic acid-type structures, which was supported by possible structures that matched the lower molecular mass range (max. 10 carbon atoms) of these DBPs. 1H-NMR before and after disinfection revealed about a 2% change of the overall 1H-NMR signals supporting a significant change of the DOM caused by disinfection. This study underlines that a large and increasing number of people are exposed to a very diverse pool of organohalogens through water - by both drinking and uptake through the skin upon contact. Non-target analytical approaches are indispensable to reveal the magnitude of this exposure and to test alternative ways to reduce it.