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ii Inactivation of bacteriophage MS2 by reactive oxygen species (ROS) produced by irradiation with simulated sunlight, containing visible light and ultraviolet light with wavelengths greater than 320 nm, was investigated. The results from experiments with synthetic sensitizers Rose Bengal, 3'methoxyacetophenone, and nitrite suggested that singlet oxygen ( 1 O2), triplet excited state of dissolved organic matter ( 3 DOM*), and hydroxyl radicals (·OH) could separately inactivate bacteriophage MS2. Natural sources of sensitizers included purified DOM isolates obtained from wastewater and river waters and three water samples collected from Singapore River, Stamford Canal, and Marina Bay Reservoir in Singapore. Linear correlations were found between MS2 inactivation rate constants (kobs) and the photo-induced reaction rate constants o