Matrix isolation infrared (IR) studies have been carried out on the vaporisation of the alkali-metal azides MN₃ (M = Na, K, Rb and Cs). The results show that under high vacuum conditions, molecular KN₃, RbN₃ and CsN₃ are present as stable high-temperature vapour species, together with variable amounts of nitrogen gas and the corresponding metal atoms. The characterisation of these molecular azides is supported by ab initio molecular orbital calculations and density functional theory (DFT) calculations, and for CsN₃ in particular, by the detection of the isotopomers Cs(¹⁴N¹⁵N¹⁴N) and Cs(¹⁵N¹⁴N¹⁴N). The IR spectra are assigned to a "side-on" (C_2v) structure by comparison with the spectral features predicted both by vibrational analysis and calculation. The most intense IR features for KN₃, RbN₃ and CsN₃ isolated in nitrogen matrices lie at 2005, 2004.4 and 2002.2 cm^-1, respectively, and correspond to the N₃ asymmetric stretch. The N₃ bending mode in CsN₃ is identified at 629 cm^-1. An additional feature routinely observed in these experiments occurred at approximately 2323 cm^-1 and is assigned to molecular N₂, perturbed by the close proximity of an alkali-metal atom. The position of this band appeared to show very little cation dependence, but its intensity correlated with the extent of sample thermal decomposition.