Chlorine dioxide is being used to control THMs formation or taste & odor in water treatment plant. Recently, some operators or academic circles doubted the effectiveness of stabilized chlorine dioxide which is presumed as a liquid form of chlorine dioxide. In this study, we investigated components which consist of stabilized chlorine dioxide in terms of chlorine dioxide and chlorite. Two analytical methods used in this study are UV method and Iodometric method. Iodometric method is recommended by Korean EPA to check the purity of stabilized chlorine dioxide. The samples of stabilized chlorine dioxide from four water treatments were investigated and compared with that produced from chlorine dioxide generator on-site. This study demonstrated that the component of stabilized chlorine dioxide was overwhelmingly chlorite (CIO_2￣) not chlorine dioxide (CIO_2) by UV method. It was also proved that Iodometric method (2nd method) recommended by Korean EPA could not differentiate between CIO_2 and CIO_2￣. Iodometric method (2nd method) recommended by Korean EPA should be revised accordingly to measure chlorine dioxide properly.