The development of chlorine containing species during the hydrogen peroxide-based chlorine dioxide generation process has been determined. Accordingly, two distinct phases, namely the induction period and the steady-state phase, were identified. In the induction period, it was observed that chloride and chlorous acid are generated, while chlorine, a byproduct from some methanol-based processes, is not detectable. The absence of chlorine is explained by the fast reaction kinetics between hydrogen peroxide and chlorine, which results in the formation of chloride. In the steady-state phase, due to the accumulation of chloride and chlorous acid during the induction period, the reaction between chloric acid and chlorous acid, which is responsible for the generation of chlorine dioxide in the hydrogen peroxide-based ClO2 process, becomes possible. Chloride is a catalyst in such a reaction.