Despite reservations of a number of researchers concerning the applicability of Bond's equation for relating specific energy to the size reduction of rocks, his approach has become an industry standard. Data from a large number of pilot and full-scale autogenous (AG), semi-autogenous (SAG) and ball mill circuits are used to show that Bond's equation does not hold over the particle size range -100 mm + 0.1 mm. However, if suitable correction factors are applied its results can be made to approximate observed values. It is shown that a more appropriate relationship relies on a particle size exponent that is a function of size. This new relationship, which does not rely on the need for correction factors, is used to predict the specific energy requirements of a number of autogenous, semi-autogenous and ball mill grinding circuits. The results are compared to those obtained using Bond's equation and are shown to provide a more precise prediction of specific power requirements. (C) 2003 Elsevier B.V. All rights reserved.