Gibbsite (AlOH)(3) was mechanically activated by grinding for 20 h, and the changes in its structure were studied by thermal analysis, X-ray powder diffraction and Al-27 MAS NMR. Grinding caused the rupture of a significant proportion of the Al-OH bonds, but the resulting molecular water was immediately adsorbed on to the activated surfaces from which it could be endothermically desorbed at 125 degrees C. Grinding causes the gibbsite to become X-ray amorphous, containing octahedral and tetrahedral sites, and another site at about 34 ppm sometimes ascribed to Al in 5-fold coordination; the relative site occupancies suggest that this phase is similar to rho-Al2O3. The amorphous phase converts via gamma-Al2O3 to alpha-Al2O3 (corundum) at 900 degrees C, by contrast with unactivated gibbsite which transforms to corundum via gamma and theta-Al2O3 at least 400 degrees C higher