Both simulations and experimental results show that a period-doubling route leads from simple pH oscillations through period 2 and period 4 cascades to chaos in an aqueous H2O2-sulfur(IV)-HCO3- flow system by varying either the removal rate of CO2 from the reaction mixture or the flow rate. Windows of stable period 3 oscillations are also found in the wide chaotic region calculated in the [H2O2](0)-[SO32-](0) plane. An autocatalytic production of H+ during the oxidation of sulfur(IV) by H2O2 is responsible for the positive feedback, while a H+ consuming negative feedback is provided by the relatively slow reversible formation of CO2 (HCO3- + H+ reversible arrow CO2 + H2O) and its controlled escape from the surface of the reaction mixture. Protonation of SO32- also contributes to the removal of the free H+. Such a multichannel negative feedback appears to be the clue to the chaotic dynamical behavior. This seems to be the simplest known chaotic chemical reaction system.