Chlorine dioxide (ClO₂) has emerged as a promising alternative to free chlorine for water disinfection and/or pre-oxidation due to its reduced yields of chlorinated disinfection byproducts. ClO₂ decomposes to form chlorite (ClO₂⁻), which influences the following advanced oxidation processes (AOPs) for micropollutant abatement in drinking water. This study aims at investigating the effects of ClO₂⁻ on the concentrations of reactive species (e.g., radicals and ozone) and on the formation of chlorate in the UV/chlorine AOP. Results showed that the concentration of ClO^· in the UV/chlorine process remarkably decreased by 98.20–100.00% in the presence of ClO₂⁻ at concentration of 0.1–1.0 mg·L⁻¹ as NaClO₂. The concentrations of HO^· and ozone decreased by 42.71–65.42% and by 22.02–64.31%, respectively, while the concentration of Cl^· was less affected (i.e., 31.00–36.21% reduction). The overall concentrations of the reactive species were differentially impacted by ClO₂⁻’s multiple roles in the process. UV photolysis of ClO₂⁻ generated HO^· but not Cl^·, ClO^· or ozone under the drinking water relevant conditions. ClO₂⁻ also competed with chlorine for UV photons but this effect was minor (< 1.0%). The radicals/ozone scavenging by ClO₂⁻ outcompeted the above two to lead to the overall decreasing concentrations of the reactive species, in consistency with the kinetic model predicted trends. ClO₂⁻ reacted with radicals and ozone to form chlorate (ClO₃⁻) but not perchlorate (ClO₄⁻). HO^· played a dominant role in ClO₃⁻ formation. The findings improved the fundamental understanding on micropollutant abatement and inorganic byproduct formation by the UV/chlorine process and other AOPs in ClO₂⁻-containing water.