The chemical and morphological changes induced on the surface of millerite by mechanical activation have been studied by means of X-ray diffraction, electron microprobe, scanning electron microscopy and infrared spectra. Leaching tests in water, dilute nitric and hydrochloric acids have been carried out. The mechanical activation of millerite grains due to their disintegration is accompanied by an increase in the number of particles and generation of fresh previously unexposed surface. The agglomeration of grains and a partial oxidation of sulphide to sulphate ions were observed. The increase in leachability of the activated millerite may well be due to the combination of mechanically induced structural defects and chemical modification of millerite surface. (C) 2002 Elsevier Science B.V. All rights reserved.