Three Southeastern United States water treatment plants using chlorine dioxide, one of the methods available for controlling trihalomethanes, are studied in detail. Treatment records are reviewed, and tests of the water are made for THM, TOX, TOC, and residual disinfectants. A treatment scheme using ClO2 as a pre-oxidant/disinfectant and Cl2 as a final disinfectant is found to give fairly equivalent water to that from the same treatment scheme using Cl2 alone. Trihalomethane concentration is greatly reduced when using ClO2/Cl2 rather than Cl2 alone. However, high chlorite concentration and microorganism regrowth in the distribution system can both be problems in poor quality waters.