An attempt is made to measure the energy stored in the material during mechanical activation and its manifestation in various forms (defects, new surfaces and interfaces, strain and structural disorder) through direct energy measurements, calorimetry, surface area and surface energy measurements. The elastic strain, grain size and extent of amorphisation is estimated from XRD analysis. The strain energy is calculated using the theory of elasticity and the amorphisation energy from thermodynamic calculations. For ilmenite, the energy transferred to the material is about 6% of the specific energy consumption in 4 h of planetary milling. A major part of the energy is stored as elastic strain energy, structural disorder and in point and line defects whereas the energy stored in additional surfaces and grain boundaries are comparatively much lower. (C) 2009 Elsevier Ltd. All rights reserved.