Boehmite (gamma-AlOOH) and gamma-Al2O3 were ground for 60 min and the changes in their structure and thermal reactions were monitored by X-ray powder diffraction, thermal analysis and Al-27 MAS NMR. Grinding does not cause the boehmite to dehydrate, but causes a decrease in the intensity of its X-ray reflections, and the formation of an amorphous phase containing tetrahedral Al and another Al site with a resonance at 36 ppm (sometimes attributed to pentacoordinated Al). The ground material thermally transforms to corundum at 940 degrees C via gamma-Al2O3, by contrast with the unground control, which forms corundum at 1195 degrees C via gamma and delta-Al2O3. Grinding gamma-Al2O3 decreases the intensity of its X-ray reflections and induces the formation of some corundum without further heating. Complete transformation to corundum occurs at 927 degrees C, compared with 1193 degrees C in the unground material which transforms via delta-Al2O3, The relationship between the mechanochemical development of the 36 ppm Al-27 NMR resonance in aluminas and their hydroxyl and adsorbed water content is discussed.