The reaction microscopic mechanism of chlorobenzene with hydrogen peroxide in aqueous solutions under different conditions was studied by laser flash photolysis. The main characteristic peaks in these transient absorbance spectra were assigned and the build-up/decay trends of several transient species were investigated. OH free radical can be produced through the 266 nm laser flash photolysis of hydrogen peroxide in aqueous solutions. The reaction of OH free radical and chlorobenzene produces C₆H₅Cl-OH adduct with a rate constant of (5.89±0.65)×10⁹ (L•mol^－1•s^－1) in the approximately neutral solution (pH≈6) or (7.07±0.61)×10⁹ L•mol^－1•s^－1 in acid (pH＝1.5) solution, while its rate constant is about (4.34±0.51)×10⁹ L•mol^－1•s^－1 in the basic (pH＝9) solution. In the approximately neutral solution (pH≈6) or in acid (pH＝1.5) solution, the intermediate can decay from itself gradually through a second-order reaction with 2k/εl＝1.1×10⁶ s^－1. While in the basic (pH＝9) solution, its decay trend shows pseudo-first-order rate constant of 2.11×10⁵ s^－1. The C₆H₅Cl-OH adduct will be able to react with O₂ to form C₆H₅Cl-OHO₂.

Key words: laser flash photolysis, chlorobenzene, hydrogen peroxide, transient absorption spectra, •, OH radical