This paper describes the structural and surface changes in gibbsite subjected to mechanical activation using stirred media mill. During milling, the median particle size decreased from 124 to around 3 mu m in 30 min indicating high milling efficiency. Scanning electron microscopy studies revealed that gibbsite particles, present in the feed as agglomerates of pseudohexagonal platelets, ruptured by attrition at grain joints followed by fracture leading to further fragmentation. During milling, progressive decrease in the intensity of the (002) peak of gibbsite in the powder X-ray diffraction pattern indicated amorphisation. It was found that the presence of hematite favoured amorphisation of gibbsite. The zeta potential and isoelectric point of gibbsite changed during milling, which indicated alteration of the gibbsite surface. (c) Koninklijke Brill NV, Leiden and Society of Powder Technology, Japan, 2008