Abstract A vacuum infrared cell for reactive or intermediate species has been devised by the use of polyethylene sheets as optical windows. The infrared spectra in the 4000–3100cm−1, 2500–1500 cm−1, 1250–750 cm−1 and 700–65 cm−1 regions can be measured with this liquid cell. The vibrational assignments of the observed spectra have been carried out. The band at 367 cm−1 has been assigned to ν17, and the band at 175 cm−1, to ν18 and ν14. The normal vibrations have been calculated, and the potential function of this molecule has been studied. A bond has been concluded to lie between two aluminum atoms. The characteristics of the potential function and the bond structure of trimethylaluminum have also been discussed.