Bromine chloride in the presence of chloride ion forms the dichlorobromate(I) ion, BrCl2-, where K-1 = [BrCl2-]/([BrCl(aq)][Cl-]) = 6.0 M(-1). Equilibrium constants (all at 25.0 degrees C, mu = 1.00 M) are also determined for K-2 = [Br2Cl-]/([BrCl(aq)][Br-]) = 1.8 x 10(4) M(-1), for K-3 = [Br2Cl-]/([Br-2(aq)][Cl-]) = 1.3 M(-1) and for K-4 = [Br-3(-)]/[Br-2(aq)][Br-] = 16.1 M(-1). UV absorption bands are resolved for BrCl2- at 232 nm (epsilon 32 700 M(-1) cm(-1)) and 343 nm (epsilon 312 M(-1) cm(-1)), for Br2Cl- at 245 nm (epsilon 24 900 M(-1) cm(-1)) and 381 nm (epsilon 288 M(-1) cm(-1)), and for Br-3(-) at 266 nm (epsilon 40 900 M(-1) cm(-1)). The UV spectral properties of Cl-2(aq), Cl-3(-), Br-2(aq), and Br- are examined and compared. The reaction between Cl-2(aq) and Br- to form BrCl2- occurs at the diffusion-controlled limit; the rate constant, (7.7 +/- 1.3) x 10(9) M(-1) s(-1), is measured by the pulsed-accelerated-flow method. The rapid formation of BrCl2- can be used as an analytical method for trace bromide ion, where as little as 10(-5) mol % Br- can be detected in aqueous solutions of HCl or chloride salts.