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Mechanical activation of industrially prepared gibbsite (with a mono modal size distribution with a median size of 110 microns) has been carried out in a planetary mill at ambient conditions. The samples were described in terms of particulate characteristics, structural characteristics and reactivity. The mono modal size distribution of the unmilled gibbsite was found to change to a broad multimodal distribution as a result of milling. Size reduction was sharp in the initial stages, with median size reducing to around 7 microns during 15 minutes of milling. The size then increased gradually with increased milling time, indicating particle aggregation. Morphological studies of milled gibbsite showed that the platelets forming the agglomerate are separated first; the separated platelets then undergo breakage. Aggregation of particles occurs in the later stages of milling as a result of increased surface area and surface energy. XRD data showed that the crystallite size of gibbsite decreases as the milling time progresses whereas its microstrain increases. TEM micrographs show signs of defects, leading to disordering of gibbsite structure. Reactivity of milled gibbsite and alkali dissolution were found to increase with mechanical activation, while dehydroxylation temperature decreased, Mechanical activation of industrially prepared gibbsite (with a mono modal size distribution with a median size of 110 microns) has been carried out in a planetary mill at ambient conditions. The samples were described in terms of particulate characteristics, structural characteristics and reactivity. The mono modal size distribution of the unmilled gibbsite was found to change to a broad multimodal distribution as a result of milling. Size reduction was sharp in the initial stages, with median size reducing to around 7 microns during 15 minutes of milling. The size then increased gradually with increased milling time, indicating particle aggregation. Morphological studies of milled gibbsite showed that the platelets forming the agglomerate are separated first; the separated platelets then undergo breakage. Aggregation of particles occurs in the later stages of milling as a result of increased surface area and surface energy. XRD data showed that the crystallite size of gibbsite decreases as the milling time progresses whereas its microstrain increases. TEM micrographs show signs of defects, leading to disordering of gibbsite structure. Reactivity of milled gibbsite and alkali dissolution were found to increase with mechanical activation, while dehydroxylation temperature decreased