Transformation of atenolol (ATN), a micropollutant containing a secondary (2°) amine moiety, can be significantly enhanced in water treatment with sequential and combined use of chlorine and UV (chlorine/UV) through photolysis of the N-Cl bond. This study investigated the transformation kinetics, products, and mechanisms of the amine moiety of ATN in chlorine/UV (254 nm). The fluence-based, photolysis rate constant for N-Cl ATN was 2.0 × 10-3 cm2/mJ. Transformation products (TPs) with primary (1°) amines were mainly produced, but TPs with 2° and 3° amines were also formed, on the basis of liquid chromatography (LC)/quadrupole-time-of-flight/mass spectrometry and LC/UV analyses. The amine-containing TPs could be further transformed in chlorine/UV (with residual chlorine in post UV) via formation and photolysis of new N-Cl bonds. Photolysis of N-Cl 1° amine TPs produced ammonia as a major product. These data could be explained by a reaction mechanism in which the N-Cl bond was cleaved by UV, forming aminyl radicals that were transformed via 1,2-hydrogen shift, β-scission, intramolecular addition, and 1,2-alkyl shift. Among these, the 1,2-alkyl shift is newly discovered in this study. Despite enhanced transformation, only partial mineralization of the ATN's amine moiety was expected, even under chlorine/UV advanced oxidation process conditions. Overall, the kinetic and mechanistic information from this study can be useful for predicting the transformation of amine moieties by chlorine/UV water treatment.