The i.r. spectra of aluminium bromide, aluminium iodide and gallium chloride vapours were measured in the region 50–700 cm −1 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 o-rings of gold. The dimer spectra were interpreted in terms of D 2 h symmetry and seven of the eight i.r. active fundamentals were assigned. The monomer spectra were interpreted in terms of D 3 h symmetry and the three i.r. active fundamentals were assigned. Valence force fields were derived for the three dimers and three monomers based upon the present i.r. frequencies and on Raman data reported previously.