The kinetics of the reactions of ClO3 − with HSO3 − and H2SO3 was studied by measuring the concentration of [Cl−] and [H+] both in chlorate-bisulfite and chlorate-sulfite/bisulfite solutions. A reaction mechanism was applied for simulation of the experimental observations. Rate constants k1 = (1±0.5)·10−4 M−1 s−1 and k2 = (0.23±0.01) M−1 s−1 were determined for the following reactions: (1) $$ ClO_3^ - + 3HSO_3^ - \to Cl^ - + 3SO_4^{2 - } + 3H^ + $$ (2) $$ ClO_3^ - + 3H_2 SO_3 \to Cl^ - + 3SO_4^{2 - } + 6H^ + $$ Rate constant k1 was obtained directly from the experimental results of chloratesulfite/ bisulfite reactions, where reaction (1) is predominant. Rate constant k2 was obtained by computer fitting of [Cl−] and [H+] to the experimental values both in chlorate-bisulfite and chlorate-sulfite/bisulfite reactions.