The reactions of triethylaluminium, ethylaluminium chlorides (Et–Al cleavage), and aluminium tribromide with isocyanates and isothiocyanates (mainly PhNCO and PhNCS) have been examined. Under mild conditions, solid 1 : 1-insertion (into Et–Al or Br–Al bonds) compounds [e.g. Br2AlN(Ph)C(:O)Br from Al2Br6 and 2PhNCO] were isolated in every instance. With Al2Br6 and either PhNCO or MeNCO metastable co-ordination compounds (formulated as RNC[graphic omitted]–l[graphic omitted]Br3, and characterised by an abnormally-high NCO asymmetric stretching frequency at ca. 2400 cm.–1) were identified as the first-formed products. The insertion compounds were dimeric in solution and are assigned an eight-membered ring structure. Exceptions are Br2AlN(Ph)C(:Q)Br (Q = O or S), which are monomeric, and are believed to be chelates, with tetra-co-ordinate aluminium.