In water treatment, ultraviolet irradiation, ozonation, and chlorination are believed to produce various active oxygen species, which seem to participate in reactions with fumic acid, pollutants and bacteria. It was, however, very difficult to determine the actual kinds and amount of active oxygen during the processes because of deficiencies of method. In order to determine the kinds and amounts of active oxygen species which may participate in water treatment process, the spin-trapping ESR (electron spin resonance) technique was applied to measuring unstable free radicals generated during chlorination and ozonation. As spin-trapping reagent, DMPO (5,5-dimethyl-1-pyrroline-N-oxide) was dissolved in phosphate-buffered water, and the solution was treated by either chlorination or ozonation. Immediately after the treatment, ESR spectrum was measured at room temperature. Typical ESR spectra due to DMPO-OH were observed from both treatments, indicating the generation of hydroxy radical in chlorination and ozonation. In addition to DMPO-OH, several different signals were observed and their appearance depended on the chlorine or ozone concentration.