Aqueous chlorine solutions (defined as chlorine solutions (Cl 2,T) containing solely or a combination of molecular chlorine (Cl2), hypochlorous acid (HOCl), and hypochlorite (OCl-)) are known to produce toxic inorganic disinfection byproduct (e.g., chlorate and chlorite) through photoactivated transformations. Recent reports of perchlorate (ClO4-) production-a well-known thyroid hormone disruptor- from stored bleach solutions indicates the presence of unexplored transformation pathway(s). The evaluation of this potential ClO4 - source is important given the widespread use of aqueous chlorine as a disinfectant. In this study, we perform detailed rate analysis of ClO 4- generation from aqueous chlorine under varying environmental conditions including ultraviolet (UV) light sources, intensity, solution pH, and Cl2,T concentrations. Our results show that ClO 4- is produced upon UV exposure of aqueous chlorine solutions with yields ranging from 0.09 × 10-3 to 9.2 × 10-3% for all experimental conditions. The amount of ClO 4- produced depends on the starting concentrations of Cl2,T and ClO3-, UV source wavelength, and solution pH, but it is independent of light intensity. We hypothesize a mechanistic pathway derived from known reactions of Cl2,T photodecomposition that involves the reaction of Cl radicals with ClO 3- to produce ClO4- with calculated rate coefficient (kClO4-) of (4-40) × 105 M -1 s-1 and (3-250) × 105 M-1 s-1 for UV-B/C and UV-A, respectively. The measured ClO 4- concentrations for both UV-B and UV-C experiments agreed well with our model (R2 = 0.88-0.99), except under UV-A light exposure (R2 = 0.52-0.93), suggesting the possible involvement of additional pathways at higher wavelengths. Based on our results, phototransformation of aqueous chlorine solutions at concentrations relevant to drinking water treatment would result in ClO4- concentrations (∼0.1 μg L-1) much below the proposed drinking water limits. The importance of the hypothesized mechanism is discussed in relation to natural ClO4- formation by atmospheric transformations.