A number of experiments have been performed in an effort to better understand the photoinitiated chain decomposition of ClN3. Discharge-flow methods were used to determine the rate of energy exchange between vibrationally excited N2 (a likely chain carrier) and ClN3. The rate constant for energy transfer from N2(v=1) to ClN3 was found to be (2.0 ± 1.0) × 10-13 cm3 s-1 at 300 K. This process is thought to excite the ν2 mode in ClN3 with the release of 281 cm-1 of excess energy. Experiments were also performed in which the decomposition of ClN3 was initiated by photolysis with a pulsed KrF laser at 249 nm, with subsequent observation of the time dependence of the densities of ClN3, electronically excited NCl(aΔ), and vibrationally excited ClN3(ν2). A kinetic model for the ClN3 decomposition was assembled based on reactions with NCl(a1Δ) and N2(ν).