The behaviors of haloacetic acids (9 species) and trihalomethanes in the process of two water purification plants at Ogasawara Islands were investigated in August 2003 and January 2004. The total haloacetic acid formation potentials (THAAFPs) in raw water were within the range of 178-356μg · l^-1 with the concentrations in January exceeding those in August. The total trihalomethane formation potentials (TTHFPs) in raw water were within the range of 149-330μg · l^-1 with the concentrations in August exceeding those in January. After coagulating sedimentation treatment, the THAAFPs and TTHMFPs were within the ranges of 33-59μg · l^-1 and 44-86μg · l^-1, respectively. The removal ratios of THAAFP tended to be higher than those of TTHMFP. THAAFP correlated closely with potassium permanganate consumption and ultraviolet absorption (260nm). Following the coagulating sedimentation treatment, the use of chlorination caused a sudden decrease in the bromide ion concentration in water and generated trihalomethanes and haloacetic acids. Almost all of these concentrations gradually increased in tap water, with brominated haloacetic acids and trihalomethanes accounting for nearly all of the total haloacetic acids and total trihalomethanes.