The formation of disinfection by-products (DBPs) during water disinfection is a health concern. A limited number of DBPs, such as trihalomethanes (THMs), are regulated and used as indicators of human exposure to the broader group of DBPs. However, it remains poorly understood whether the formation mechanisms and precursors of unregulated DBPs are similar to that of regulated ones. In this study, lab-scale chlorination and chlorine dioxide (ClO₂) disinfection were conducted on four different source waters (two surface water and two groundwaters). DBP formation was assessed through targeted analysis of THMs via gas chromatography-mass spectrometry, novel sulfonated DBPs via supercritical fluid chromatography-mass spectrometry, and non-targeted analysis using liquid chromatography coupled to Fourier transform ion cyclotron resonance mass spectrometry (LC-FT-ICR-MS). The formation of THMs during chlorination was higher in surface waters (49-111 µg per mg dissolved organic carbon, DOC, at 48 h) than in groundwaters (21-27 µg per mg DOC) and corresponded to their higher initial specific UV-absorbance (SUVA₂₅₄) and higher humic acid fractions as determined by LC-organic carbon detection. ClO₂ disinfection led to significantly lower THM levels (below the limit of detection of 0.20 µg/L) across all samples. Similarly, the formation of sulfonated DBPs was one order of magnitude lower. However, unlike THMs, sulfonated DBPs were formed to a greater extent in both groundwaters (3.0-3.6 µg per mg DOC) than in surface waters (2.0-2.2 µg per mg DOC), suggesting that sulfonated DBPs are preferentially formed from other precursors than THMs. This was further elucidated by LC-FT-ICR-MS analysis showing that the higher levels of sulfur- and nitrogen- containing dissolved organic matter in groundwaters likely contributed to the increased formation of sulfonated DBPs. Furthermore, LC-FT-ICR-MS analysis outlined that disinfection by ClO₂, while reducing halogenated DBPs, resulted in even higher levels of non-chlorinated, sulfur- and nitrogen-containing DBPs. In conclusion, strategies focused on reducing regulated THMs may be insufficient to mitigate the formation of sulfonated and other novel heteroatom-containing DBPs during drinking water treatment.