It has been clearly shown that not only tetrathionate but also pentathionate or even higher polythionates is formed in the oxidation of thiosulfate by hypochlorous acid in a slightly acidic medium. In thiosulfate excess, the absorbance-time curves registered at 250 nm may go through a maximum followed by a minimum, suggesting the presence of a short-lived absorbing intermediate proposed to be SI-03 OC13-. Matrix rank analysis (MRA) studies have revealed that altogether five independent absorbing species are present in the wavelength ran-e 240-400 nm. A kinetic model is suggested to take all of the experimental observations into account.