To study the radiation-generated ROS in aqueous solution containing organochloride, we used 2,2,2-trichloroethan-1-ol (TCE) as the model organochloride since TCE is miscible with water and is the most efficient among organochlorides tested in the previous work. 4-(Methylthio)phenol 1 (Figure ) was used as the reductant. Irradiation of compound 1 in deuterated water (D2O)/TCE results in the formation of new peaks on proton nuclear magnetic resonance ( 1 H NMR) (Figure ), which can be assigned to 4-(methylsulfinyl)phenol 2. In contrast, irradiation of compound 1 in D2O (no TCE added) resulted in the same NMR spectrum as before irradiation (Figure ), indicating that compound 1 is not reactive to typical species formed by water radiolysis such as hydroxyl radical and hydrogen peroxide. Compound 1 (50 µM) in D2O/12 mM TCE was exposed to increasing dose of gamma radiation. As shown in Figure , the yield of compound 2 showed a linear increase with administered dose from 2 to 60 Gy, and the oxidation reached full conversion within 90 Gy of radiation (Figure ). The yield of compound 2 per received dose was 0.64 µM/Gy. It should be noted that the G-value (yield of species generated per received dose) of aqueous electrons is 0.28 µM/Gy. The higher yield of compound 2 suggests that the reaction of aqueous electrons and TCE can compete with the spur reactions thereby causing more aqueous electrons to be converted to peroxylradicals. We then irradiated compound 1 (50µM) to reach 60 Gy in D2O at various concentrations of TCE and calculated the yield per received dose at different conditions. As shown in Figure , the yield of compound 2 decreased when less TCE was applied, from 0.68 µM/Gy at 24 mM TCE to 0.054 µM/Gy at 60 µM of TCE. The phenomenon that the yield of oxidising product depends on the concentration of organochloride was also observed in the previous work, where more chloroform resulted in more stilbene oxidation. The formation of the organochloride peroxyradical intermediate (M2 in Scheme 2) by irradiating an aqueous solution containing organochloride follows a series of reaction steps: radiolysis of water produces aqueous electrons which react with the organochloride to produce chloride ion and a carbon-centered radical. In aerated solutions, the carboncentered radical reacts with molecular oxygen to produce the peroxyl radical. However, aqueous electrons can also react with dissolved molecular oxygen to form peroxyl radical anion that has been reported to be unreactive toward organochlorides. In the previous experiments, the concentration of oxygen was constant and should be approximately 256 µM. We hypothesized that the competition of aqueous electrons reacting with either molecular oxygen or the organochloride radical made the organoperoxyradical generation rely on concentrations of organochloride and oxygen. Although higher oxygen concentration will inhibit the reaction of aqueous electrons with organochloride, oxygen is essential at the organoperoxyradical generation pathway. So, the molar ratio of organochloride and oxygen is likely to exhibit an optimal value, which is difficult to predict without calculation. Scheme 2. Proposed reaction network of the oxidation of compound 1 by organoperoxyradical generated from irradiating an aqueous solution containing organochloride and molecular oxygen.