Disinfection processes such as chlorination, chloramination, and ozonation are a crucial measure to provide safe drinking water because they effectively inactivate pathogenic microorganisms. However, these processes can also lead to the formation of harmful disinfection by-products (DBPs) such as the carcinogen N-nitrosodimethylamine (NDMA). NDMA is primarily formed during chloramination of wastewater and wastewater-impacted surface waters, where natural organic materials and anthropogenic pollutants serve as NDMA precursors. A better understanding of NDMA precursors and various formation pathways is vital for developing appropriate mitigation strategies. Compound-specific isotope analysis (CSIA) can be used to both allocate sources of organic contaminants in the environment and elucidate their (trans)formation pathways. The goal of this dissertation was to explore the use of CSIA for assessing NDMA formation mechanisms and identifying reactive precursor moieties from stable isotope fractionation trends. In laboratory model systems, NDMA formation was studied during chloramination of secondary and tertiary amines, which are relevant precursor compounds in natural waters. Reaction kinetics and stoichiometries were determined to elucidate poorly characterized reaction steps of the NDMA formation pathway involving chloramine, molecular oxygen, and intermediate species. Although molar NDMA yields from the selected precursors differed significantly (1% - 90%), one O2 molecule was consumed per N(CH3)2 moiety of the precursor. This observation indicates that the reaction of O2 with secondary and tertiary amines proceeded via the same mechanism, but did not control the molar NDMA yield. NDMA formation coincided with the disappearance of the precursor, demonstrating that (oxygen) intermediates were highly reactive and short-lived. Changes of 18O/16O isotope ratios in aqueous O2 revealed that oxygen reacted with radical species, which was confirmed by additional experiments with radical scavengers (tert-butanol, ABTS, and trolox). Based on these results, a NDMA formation mechanism was proposed involving N-centered aminyl radicals. To investigate whether changes of the natural isotopic composition of NDMA can provide additional insights into the NDMA formation mechanism, an analytical method for the accurate determination of C, H, and N isotope ratios of NDMA was developed using solid-phase extraction coupled to gas chromatography isotope ratio mass spectrometry (SPE-GC/IRMS). During chloramination of tertiary amines, C and H isotope ratios of NDMA remained widely unaltered. Along with quantitative deuterium nuclear magnetic resonance spectroscopy, this result demonstrated that the N(CH3)2 moiety of NDMA originated from the tertiary amine precursor. In contrast, the N atom of the nitroso group of NDMA stemmed from NH2Cl as inferred from experiments with 15N-enriched NH2Cl. N isotope ratios of NDMA increased significantly during its formation meaning that 14N reacted preferentially to NDMA. Several steps of the reaction sequence leading to NDMA can be responsible for this observable N isotope fractionation. Trends in correlated C and N isotope ratios of NDMA were nevertheless characteristic for chloramination of four tertiary amines and might serve as probes for this class of precursors. This important proof-of-concept work is a first step towards applying CSIA to reveal relevant NDMA precursors in water treatment processes.