The chlorous acid reaction with hypochlorous acid (chlorine) may be responsible for the chlorine dioxide formation in a methanol based chlorine dioxide generation process. In this paper, the reaction between chlorous acid and hypochlorous acid in a 4.5 mol/L sulfuric acid solution was studied. It was found that chlorine dioxide is generated only if chloride is present. The underlying mechanism associated with the effect of chloride on the reaction was discussed.