A comminution result is determined by the number of stress events and the stress intensity. Against that, the comminution behavior of a mill is characterized by the frequency of stress events and the stress energy acting at each stress event. The product of stress number and stress energy is proportional to the specific energy input. The proportionality factor is the energy transfer factor which describes the percentage of energy which is used for stressing the product particles. Using the characteristic numbers based on a few tests, the operating parameters can be optimized. By comparing the mill characteristics with the breakage characteristics of the feed material, the performance of different mills and different operating states can he evaluated. (C) 2004 Published by Elsevier B.V.