Cavity ring-down (CRD) techniques were used to study the kinetics of the reaction of Br atoms with ozone in 1-205 Torr of either N 2 or O 2 , diluent at 298 K. By monitoring the rate of formation of BrO radicals, a value of k(Br ϩ O 3 ) ϭ (1.2 Ϯ 0.1) ϫ 10 Ϫ12 cm 3 molecule Ϫ1 s Ϫ1 was established that was independent of the nature and pressure of diluent gas. The rate of relaxation of vibrationally excited BrO radicals by collisions with N 2 and O 2 was measured;The increased efficiency of O 2 compared with N 2 as a relaxing agent for vibrationally excited BrO radicals is ascribed to the formation of a transient BrO-O 2 complex.