Sustained oscillations in pH and redox potential are found in the chlorite-sulfide reaction in a continuous-flow stirred tank reactor (CSTR). Autocatalytic oxidation of HSO3- by ClO2- is the major source of positive feedback of hydrogen ions. The reaction between H2S and ClO2- to form S-8, which consumes H+, is an important source of negative feedback. A model consisting of five protonation-deprotonation equilibria and nine redox reactions is proposed for the oscillatory reaction between S2- and ClO2-. The 10 species included are HS-, H2S, S2O32-, HSO32-, HSO3-, OCl-, HOCl, ClO2-, H+, and OH-. In contrast to the H2O2-S2- oscillatory reaction, S2O32- is shown here by capillary electrophoresis to be an important intermediate. Simulations give qualitative agreement with the pH oscillatory behavior observed in the CSTR.