3-Azidopropene, CH 2CH-CH 2-N 3, has been synthesized by a safe, small-scale procedure applicable also to other azides. IR spectra have been recorded of the liquid, the amorphous solid, and the matrix isolated species in N 2 and Ar at 14 K. Raman spectra of the liquid at different temperatures and of the amorphous and crystalline solids were also obtained. Fully optimized geometries for the five distinct conformations of 3-azidopropene have been calculated at the SCF level employing a basis set of double zeta quality and compared with the recent results from electron diffraction. The calculations show that the GG conformation is the most stable, followed, in decreasing order of stability, by SG (+2.1 kJ mol −1), SA (+5.3 kJ mol −1), GG' (+6.4 kJ mol −1) and GA (+6.5 kJ mol −1), where the capital letters refer to the conformation around the C-C and C-N bonds respectively. The IR and Raman spectral data show the GG to be the most stable conformer both in the N 2 and Ar matrices, as well as in the liquid. Whereas the low-temperature crystalline solid is made up of molecules in the SG conformation. Apparently, all five conformers are observed in the liquid at room temperature and trapped in the Ar matrix at 14 K, whereas only three conformers (GG, SG and GG') were observed in the N 2 matrix. In the liquid phase the enthalpy difference between the GG and SG conformations was found tobe Δ H° GG→SG = 1.3(3) kJ mol −1. The corresponding results for the SA and GA conformations were 5.2 (22) and 4.4(7) kJ mol −1 respectively, and Δ H° GG→GG' was estimated as 3.6 kJ mol −1.