The distribution of cationic vacancies in transition aluminas obtained by dehydration of boehmite has been investigated using HRTEM combined with an image simulation technique. Interstitial vacancies in a cubic-packed oxygen network have been found to be concentrated in an orderly fashion on low indexed lattice planes like the {110} and {100} planes of a spinel structure. Such arrangements of cationic vacancies on octahedral sites result in modified transition aluminas with lattice parameters a=a(gamma), b=a(gamma) or 2a(gamma), c=3a(gamma), where a(gamma)=0.79 nm is the lattice constant of the spinel structure of gamma-Al2O3.