The thermal behavior of mechanochemically activated sphalerite during aging was investigated by calorimetry. The results indicate that mechanochemically activated sphalerite releases the stored energy which may origin from a series of complex transformations. The amount of energy released increases with the grinding time but remains almost constant after grinding for I h. It is independent of the grinding atmosphere and is related to the ball-mill medium. The XRD results illustrate a difference of the microstructure between activated and non-activated sphalerite. The microstructure of the activated sphalerite remains the same when it is heated in a calorimeter for the measurement of the heat released. The particle size analyses show that the particle size of the activated sphalerite increases when it is heated in the calorimeter. Therefore, it can be concluded that the released energy is probably caused by the decrease of the specific surface energy. (C) 2003 Elsevier Science B.V. All rights reserved.