The dehydration products of aluminium hydroxides have been studied in vacuo. Observations made by X-ray diffraction and gas adsorption have been compared with those of other authors. Notable points were the formation of nearly amorphous oxide from trihydroxide II and the absences of the tetragonal oxide; of oxide monohydroxide in the dehydration of trihydroxide I; and of the χ–κ sequence in the dehydration of trihydroxide II. Sintering in the system is discussed.