The combined ultraviolet (UV) and free chlorine (UV–chlorine) advanced oxidation process that produces highly reactive hydroxyl radicals (HO•) and chlorine radicals (Cl•) is an attractive alternative to UV alone or chlorination for disinfection because of the destruction of a wide variety of organic compounds. However, concerns about the potential formation of chlorinated transformation products require an understanding of the radical-induced elementary reaction mechanisms and their reaction-rate constants. While many studies have revealed the reactivity of oxygenated radicals, the reaction mechanisms of chlorine-derived radicals have not been elucidated due to the data scarcity and discrepancies among experimental observations. We found a linear free-energy relationship quantum mechanically calculated free energies of reaction and the literature-reported experimentally measured reaction rate constants, kexp, for 22 chlorine-derived inorganic radical reactions in the UV–chlorine process. This relationship highlights the discrepancy among literature-reported rate constants and aids in the determination of the rate constant using quantum mechanical calculations. We also found linear correlations between the theoretically calculated free energies of activation and kexp for 31 reactions of Cl• with organic compounds. The correlation suggests that H-abstraction and Cl-adduct formation are the major reaction mechanisms. This is the first comprehensive study on chlorine-derived radical reactions, and it provides mechanistic insight into the reaction mechanisms for the development of an elementary reaction-based kinetic model.