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 k(1) =(1+/-0.5).10(-4) M-1 s(-1) and k(2) = (0.23+/-0.01) M-1 s(-1) were determined for the following reactions: ClO3- + 3HSO(3)(-) -> Cl- + 3SO(4)(2-) + 3H(+) (1) ClO3- + 3H(2)SO(3) -> Cl- + 3SO(4)(2-) + 6H(+) (2) Rate constant k(1) was obtained directly from the experimental results of chloratesulfite/bisulfite reactions, where reaction (1) is predominant. Rate constant k(2) was obtained by computer fitting of [Cl-] and [H+] to the experimental values both in chlorate-bisulfite and chlorate-sulfite/bisulfite reactions.