The system composed of BrO3-, SO32-, and HCO3- in acidic aqueous solution has been found to exhibit pH oscillations. The state diagram spanned by the initial concentrations of BrO3- and HCO3- has been determined to exhibit a tristability among the acidic and neutral steady states and the oscillatory state, in addition to several bistability regions. It is suggested that the competition between pK(a) of H2CO3 and the decrease of pH in the neutral steady state due to the increase of initial concentration of BrO3- plays a crucial role in the realization of the complicated state diagram and multistabilities observed.