This study was designed to elucidate the behavior of chlorine dioxide in drinking water systems. Furthermore, the factors that influence the formation of chlorite, chlorate in terms of reaction time, concentration of chlorine dioxide, pH, temperature and UV irradiation were experimentally reviewed. At 20 ℃, pH 7, 70-80% of chlorine dioxide injected was converted to chlorite and 0-10% of that was transformed into chlorate within 120 min with 2.91 mg/L of DOC. The amount of chlorite formed also increased when pH and temperature increased. As DOC content increased, the residual chlorine dioxide decreased but the amount of chlorite and chlorate were increased. These experiments revealed that chlorate was a dominant by-product under UV irradiation. The models that were obtained by the regression analysis for the formation of chlorite and chlorate from chlorine dioxide with Han River water are as follows: Chlorite (mg/L)=10^-2.20[ClO₂]^0.45[pH]^0.90[temp]^0.27[TOC]^1.04[time]^0.20, Chlorate (mg/L)=10^-2.61[ClO₂]^1.27[pH]^-0.50[temp]^1.28 [TOC]^0.31[time]^0.12