Chlorine dioxide is used as both a pre-oxidant and/or a post-disinfectant in several water treatment plants in the United States. Chlorine dioxide is associated with its byproducts chlorite and chlorate. Chlorine dioxide, chlorine, chlorite and chlorate were sampled in four distribution systems where chlorine dioxide is used for disinfection purposes: Charleston, WV, Columbus, GA, New Castle, PA, and Skagit, WA. The fate of chlorine dioxide and its by-products in distribution systems is discussed. A constituent transport model (TRAK) was applied to New Castle, PA distribution systems to assess times of travel. No relationship was found between the concentrations of chlorine, chlorine dioxide, chlorite and chlorate and the computed travel times. Water treatment plant and distribution system data received from Galveston, TX where the use chlorine dioxide has being tested is analyzed and discussed. Median chlorine dioxide concentrations are relatively constant in distribution systems with a value of approximately 0.2 mg/L; however, chlorine dioxide dosages applied at the treatment plant can induce different concentrations in the distribution system. Median chlorite concentrations in distribution systems range from 0.5 to 0.8 mg/L while median chlorate concentrations are generally lower in a range between 0.1 to 0.3 mg/L. The effects of distribution systems skeletonization in constituent transport modeling are also presented. Skeletonization does not affect significantly computed times of travel when the median percentage of constituent has to be detected. However, depending on the layout of each distribution system, small variations can be observed. / Master of Science