The electronic structure, vibrational properties and pyrolysis behaviour of 3-azidopropionitrile (3APN, N3CH2CH2CN) are investigated using ultraviolet photoelectron spectroscopy (UVPES) and matrix-isolation infrared (IR) spectroscopy, supported by results from ab initio and DFT electronic structure calculations. 3APN is found to have an HOMO vertical ionisation energy (VIE) of 10.05 +/- 0.02 eV and the strongest IR absorption bands located at 2173 and 1296 cm(-1). Pyrolysis of the title compound revealed the presence of N-2, HCN, CH3CN and ketenimine, H2C=C=NH, among the decomposition products, the latter behaving as a reaction intermediate. From G3 computational results, 3-iminopropionitrile, CN-CH2CH=NH, was found to be formed in competition with H2C=N - CH2CN, through 160-180 kJ mol(-1) energy barriers. On the basis of experimental evidence and G3 results, ketenimine is proposed as the dominant intermediate arising from further decomposition of both imine (CN-CH2CH=NH) and H2C=N CH2CN compounds.