We propose an UV/chlorine advanced oxidation process using chlorine production by the anodic oxidation of chloride ions (UV/electro-chlorine process). The UV/electro-chlorine process developed steadily degraded 1,4-dioxane. The advanced oxidation performance of the UV/electro-chlorine process without pH control deteriorated with an increase in the initial pH. An increase in pH observed during the UV/electro-chlorine process was due to the accumulation of free chlorine. As the pH increased from the initial pH of 4.1 or higher, the solution pH exceeded the pKa of hypochlorous acid (HOCl), which decreased the advanced oxidation performance and enhanced the formation of chlorine-centered oxyanions of chlorite and chlorate (ClOx-). To maintain a pH lower than the pKa of HOCl, a reagent-free pH control system, involving switching on and off the electrochemical chlorine production at a threshold of pH 6.1, was introduced into the UV/electro-chlorine process. When the UV/electro-chlorine process with pH control was operated at the initial pH of 4.3 or higher, the 1,4-dioxane degradation rate was inhibited by the shortage of free chlorine production. However, the degradation rate at an initial pH of 4.2 or lower was enhanced because of a higher percentage of HOCl to free chlorine and a higher contribution of center dot Cl-2(-) to 1,4-dioxane degradation. The energy consumption rate of 1,4-dioxane removal and the molar ratio to ClOx- formation were improved by the pH control at the same initial pH range.