Gibbsite [-gamma-Al(OH)(3)] precipitated from Bayer leach liquor is mechanically activated in a planetary mill at ambient conditions. Mechanical activated (MA) gibbsite is characterized in terms of particulate characteristics, structural characteristics and reactivity. Physicochemical changes during MA involved (1) size reduction in the initial stages followed by particle aggregation in the later stages and changes in particle size distribution, and (2) progressive decrease in crystallite size, and increase in microstrain leading to eventual amorphisation. Reactivity of milled gibbsite (measured in terms of (1) decrease in its dehydroxylation temperature, and (2) increase in alkali dissolution) is found to increase with mechanical activation. Increased reactivity of milled gibbsite is correlated with various physicochemical characteristics. (C) 2014 Elsevier B.V. All rights reserved.