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The kinetics of decomposition of aqueous chlorous acid has been reinvestigated at pH 0.7-1.9, ionic strength 1.0 M (HSO 4 -/SO 4 2-), and temperature 25.0 ( 0.1°C. Optical absorbances were collected in the 240-450 nm wavelength range for up to ∼90% decomposition for time series lasting as long as 2 days. The number of absorbing species was investigated by matrix rank analysis; no absorbing intermediate was formed in significant concentration during the decomposition. Of the many mechanistic models tested, the one that fit best included the following reactive intermediates: HOCl, explains the variation in stoichiometric ratio as well as the maximum observed in the initial rate of ClO 2 formation as a function of pH. The kinetics of chlorous acid decomposition cannot be quantitatively fit through the last stages of the reaction without postulating a first-order decomposition. Scission of chlorous acid to give short-lived hydroxyl and chlorine-(II) monoxide is a plausible route for this process. A set of best-fit and literature-derived parameters is presented for the complete mechanism.