Cl• and Cl2•- radicals contribute to the degradation of trace organic contaminants (TrOCs) such as pharmaceutical and personal care products and endocrine-disrupting chemicals. However, little is known about their reaction rate constants and mechanisms. In this study, the reaction rate constants of Cl• and Cl2•- with 88 target compounds were determined using laser flash photolysis. Decay kinetics, product buildup kinetics, and competition kinetics were applied to track the changes in their transient spectra. Cl• exhibited quite high reactivity toward TrOCs with reaction rate constants ranging from 3.10 × 109 to 4.08 × 1010 M-1 s-1. Cl2•- was less reactive but more selective, with reaction rate constants varying from <1 × 106 to 2.78 × 109 M-1 s-1. Three QSAR models were developed, which were capable of predicting the reaction rate constants of Cl2•- with TrOCs bearing phenol, alkoxy benzene, and aniline groups. The detection of Cl•-adducts of many TrOCs suggested that Cl• addition was an important reaction mechanism. Single electron transfer (SET) predominated in reactions of Cl• with TrOCs bearing electron-rich moieties (e.g., sulfonamides), and their cation radicals were observed. Cl• might also abstract hydrogen atoms from phenolic compounds to generate phenoxyl radicals. Moreover, Cl• could react with TrOCs through multiple pathways since more than one transient intermediate was detected simultaneously. SET was the major reaction mechanism of Cl2•- reactions with TrOCs bearing phenols, alkoxy benzenes, and anilines groups. Cl2•- was found to play an important role in TrOC degradation, though it has been often neglected in previous studies. The results improve the understanding of halogen radical-involved chemistry in TrOC degradation.