The infrared spectra of aluminium chloride vapour (AI2CI6 and AIC13) including Aii5C16 and Ali7Clti at 473-843 K were measured in the region 700-50 cm-' with an evacuable Fourier transform spectrometer by transmission and emission techniques. Evacuable cells of nickel were employed having windows of type IIa diamond and sealed with gold O-rings. Raman spectra of AI,C16 vapour at cu 500 K and of AIC13 at 1075 K with pressures from 0.3 to 10 atm were recorded, polarization measurements were carried out and additional spectra of the 35CI and 37CI compounds were obtained. The dimer spectra were interpreted in terms of DZh symmetry. All the nine Raman-active and seven of the infrared-active dimer fundamentals were assigned. The IR active mode ulo(B1.), expected below 40 cm-', and the inactive mode u,(A.) were left unassigned. The monomer spectra were interpreted in terms of D f h symmetry and all four fundamentals were assigned. A dimer force field, involving 12 independent force constants, was derived. The calculated frequencies were fitted with a least-squares programme to 30 observed fundamentals of AI:'C1, and AlFCl6 and 14 additional isotopic shifts transferred from previous matrix isolation spectra. A complete force field for the monomer was derived, using the four observed fundamentals and seven isotopic shifts transferred from previously reported matrix isolation spectra.