BCl2N3 was isolated in a low-temperature argon matrix, and its FTIR spectrum was measured. Ab initio calculations were carried out to support the identification of the IR spectrum, and splitting due to the natural abundances of B-10 and B-11 was observed. The experimental infrared frequencies and assignments are as follows: 2170 cm(-1), the N-3 asymmetric stretch; 1350 cm(-1), the BN stretch; 1053 cm(-1), the N-3 symmetric stretch; 968 cm(-1), the BCl stretch. Upon UV photolysis, the BCl2N3 peaks decreased and new peaks at 2034 and 2087 cm(-1) with relative integrated intensities of 4:1 appeared. Ab initio calculations were carried out and support the assignment of these features to the BN stretches in the B-10 and B-11 isotopomers of CIBNCl. This intermediate is consistent with the photolytic intermediate generated during the photolysis of Bi(N-3)(3), and the photolytic mechanism is discussed.