A study of the electronic structures of AlMe3, Al2Me6, AlMeH2 and some isomers of Al2Me2H4 has been made by the S.C.M.O. method. Basis sets of 3s, 3p and diffuse and contracted 3d orbitals plus all valence electrons were employed for the aluminium atoms. The 3d orbitals stabilize the dimeric species by direct, cross-ring σ, π and δ Al—Al interactions: however, those of π type appear to be the most effective. Participation of the 3d orbitals in bridge bonding is more limited. The conformation of Al2Me2H4 which seems to be energetically the most favoured is that in which the hydrogen atoms form the bridges between the monomer units whilst the methyl groups are terminal and lie mutually trans. When the methyl group acts as a bridge, as in Al2Me6, it is expected that the two groups of three hydrogen atoms will lie in the eclipsed configuration.