Alkaline decomposition of pentathionate has been investigated by high-performance liquid chromatography, which allows us to track different sulfur-containing species simultaneously in the presence of a carbonate/hydrogen carbonate buffer at 25.0 +/- 0.1 degrees C and at a constant ionic strength. It has been shown that, besides the major product, thiosulfate, not only tetrathionate but also hexathionate appears in significant amounts during the course of the reaction. At higher pHs, both of them are unstable long-lived intermediates because of their well-known alkaline degradation, but a decrease of the pH increases their stability (especially that of tetrathionate), meaning that they may even become end products. On the basis of these observations, a 10-step kinetic model with five fitted and five fixed rate coefficients is suggested and discussed to take all of the most important characteristics of the decomposition into account. We have also demonstrated and discussed that the well-known thiosulfate-assisted rearrangement of pentathionate leading to hexathionate might only play a very minor role in the formation of hexathionate under our experimental conditions.