The assignment of group frequencies in the i.r. spectra of derivatives of aluminium borohydride has been made by the use of aluminium borodeuteride. Variations in the spectra with the change of ligand are discussed. The properties of the compounds in solutions have been studied using 1H n.m.r. spectroscopy, and data obtained over a range of temperature indicate that the 1 : 1 adducts with trimethylamine and trimethylphosphine dissociate in solution, and probably exist in equilibrium with both the 1 : 2 adducts, e.g. Al(BH4)3,2NMe3 and free aluminium borohydride.