The kinetics and stoichiometry of the reaction between hypochlorous acid and chlorous acid was studied under mildly acidic conditions (pH = 2.5-4.7). The experiments were conducted in dilute solutions (10(-4)-10(-3) M) at T = 5-25 degrees C. External buffer compounds were not used. Chlorous acid, chlorine dioxide, and chlorate concentrations were monitored with titrimetric methods. Rate coefficients and activation energies were determined for the five main steps of the kinetic model (others were adopted from literature). Both hypochlorous acid and chlorine were found to react at air appreciable rate with chlorous acid/chlorite. Chloride ion was found to promote both the overall reaction rate and chlorine dioxide/chlorate ratio. The results may he exploited in optimizing the red-ox efficiency of technical chlorine dioxide bleaching processes.