Stock solutions of micropollutants with low water solubility are commonly prepared using organic solvents in laboratory studies on degradation of these organic compounds. Dilution of the stock solution unavoidably introduces a small amount of organic solvent into the experimental working solutions. This could possibly affect the estimation of the degradation rate constants of these organic micropollutants by UV-based advanced oxidation process such as UV/H2O2. To dertionstrate this problem, the effect of organic solvents on the reaction rate constant of malathion has been investigated in the UV/H2O2 process at the concentration levels that would likely be derived from stock solutions. Several organic solvents commonly used for stock-solution preparation were selected, including acetonitrile (ACN), acetone (Ac), methanol (MeOH), ethanol (EtOH), 1-propanol (PrOH), 1-butanol (BuOH) and 1-pentanol (PeOH). The results show that the reaction rate of malathion in the UV/H2O2 process could be affected by the presence of these organic solvents, even at a concentration well below that possibly introduced during the preparation of working solutions from the organic solvent stock solutions (e.g. 0.00005%, v/nu). The suppressive effect on the reaction rate constant depends on the type and concentration. With the increase of organic solvent concentration, the reaction rate constant of UV/H2O2 gradually decreases to the value for photolysis alone. The organic solvents having a stronger reaction activity with 'OH tend to impose a greater effect on the reaction rate constant. The findings here provide a plausible explanation for the discrepancies in the rate constants reported in the literature for some organic micropollutants during the UV-based advanced oxidation processes. (C) 2016 Elsevier B.V. All rights reserved.