B(N-3)(3) was isolated in a low-temperature argon matrix and its FTIR spectrum measured. The spectrum was consistent with the known gas-phase spectrum, and splitting due to the natural abundances of B-10 and B-11 was observed. Upon UV photolysis, new peaks at 2100, 1861, and 1803 cm(-1) assigned to the linear molecule NNBN appeared as the B(N-3)(3) peaks decayed. Geometry optimizations and calculations of the ground-state frequencies for B-10 and B-11 isotopomers B(N3)3 [MP2/6-31G(d)] and NNBN [CCSD(T)/6-311G"] are reported, and the frequencies are compared to those observed in the IR spectra. The photolytic decomposition mechanism of B(N-3)(3) is discussed.