Chlorine dioxide (ClO₂) may be considered by utilities as an alternative disinfectant to ozone and chlorine to comply with disinfection requirements set by the Surface Water Treatment Rule and to control disinfection byproduct (DBP) formation. However, very little is known regarding the formation of biodegradable organic matter (BOM) by ClO₂ and its effect on the distribution system. Pilot-scale testing was conducted to determine the potential for ClO₂ preoxidation to produce BOM while treating blends of California State Project water and Colorado River water. Preoxidation was followed by conventional treatment (coagulation, flocculation, sedimentation) and parallel filtration processes (chlorinated and biologically active). The addition of ClO₂ increased the formation of formaldehyde, carboxylic acids, and assimilable organic carbon (AOC). Carboxylic acid and AOC levels following ClO₂ were generally lower when compared to previously conducted work with ozone, whereas formaldehyde levels were higher. Biofiltration reduced the level of BOM. Annular reactors (ARs) were placed downstream of filtration to simulate water flow through a distribution system (with a detention time of 6 h). Biomass measurements in the AR following ClO₂ and biological filtration were similar to measurements taken in the AR following conventional chlorine treatment. Includes 16 references, table, figures.