Freshly prepared crystalline aluminium isopropoxide contains a mixture of trimeric and tetrameric molecules. Pure tetramer is readily obtained from this mixture by removing the trimer with pyridine. The p.m.r. spectra of pyridine solutions of the trimer show that strong solvent interactions occur, and that another form, possibly the dimer, is produced at high pyridine concentrations. The structures of the various forms are discussed. © 1969.