Chloride ion catalyzes the reactions of HOBr with bromite and chlorite ions in phosphate buffer (p[H+] 5 to 7). Bromine chloride is generated in situ in small equilibrium concentrations by the addition of excess Cl- to HOBr. In the BrCl/ClO2- reaction, where ClO2- is in excess, a first-order rate of formation Of ClO2 is observed that depends on the HOBr concentration. The rate dependencies on ClO2, Cl-, H+, and buffer concentrations are determined. In the BrCI/BrO2- reaction where BrCl is in pre-equilibrium with the excess species, HOBr, the loss of absorbance due to BrO2- is followed. The dependencies on Cl-, HOBr, H+, and HPO42- concentrations are determined for the BrCl/BrO2- reaction. In the proposed mechanisms, the BrCl/ClO2- and BrCI/BrO2- reactions proceed by Br transfer to form steady-state levels of BrOClO and BrOBrO, respectively. The rate constant for the BrCl/ClO2- reaction Br) is 1.9 X 10(5) M-1 S-1. - case, (k(2)(Cl)) is 5.2 x 10(6) M-1 s(-1) and for the BrCl/BrO2- reaction (k(2)(Br)) In the BrCl/ClO2 BrOClO reacts with ClO2- to form two ClO2- radicals and Br-. However, the hydrolysis of BrOBrO in the BrCl/ BrO2- reaction leads to the formation of BrO3-and Br-.