Well-crystallized alumina monohydrates, two boehmites and a diaspore, were ground in air for 0-24 h in a planetary ball mill. The effect of grinding on the texture, structure and thermal behaviour of dehydration and alpha-transformation was examined by means of XRD, TG, DTA, SEM and Al-27 MAS-NMR. It was found that after 24 h of grinding, the boehmites changed entirely to an amorphous hydrated alumina, part of which transformed to alumina by an additional mechanochemical dehydration, while diaspore did not.