The reactivity of bromine azide, BrN 3 , towards elemental bromine, Br 2 , was studied experimentally in the gas phase and in CH 2 Cl 2 solution. The gas phase IR spectra of a 1:10 mixture of BrN 3 and Br 2 are in accord with the formation of small amounts of nitrogen tribromide, NBr 3 but it was not possible to isolate the NBr 3 from the reaction mixture. The structure and vibrational data of NBr 3 have been computed ab initio at RHF and at electron correlated MP2 and MP4(SDQ) levels of theory using a 6-31G * basis set for nitrogen and a quasi-relativistic pseudopotential for bromine (LANl2DZ). The computed (MP4(SDQ)/LANL2DZ) structural parameters are: d(N--Br) = 2.025 9, (BrNBr) = 107.7°. The observed IR data (ν 1 350, ν 3 530 cm - 1 ) are in good agreement with those calculated at MP2/LANL2DZ level of theory (ν 1 395, ν 2 203, ν 3 526, ν 4 145 cm - 1 ). 1 4 N NMR spectroscopy revealed that in solution at or below room temperature BrN 3 does not react with Br 2 to form NBr 3 . However, above -40°C BrN 3 reacts with Br 2 to form a weak complex of the type {Br + ( 2 n + 1 ) N - 3 }. ( 1 4 N NMR, CH 2 Cl 2 , rel. to MeNO 2 . Br--N a --N b --N c : - 142 (N b ), - 178 (N c ), - 328 (N a ) ppm; {Br + ( 2 n + 1 ) N - 3 }: - 132 (N a and N c ), - 304 (N b ) ppm).