Comminution efficiency is a technical term that relates some measure of the output from a comminution machine to the energy input into it. The term has usually been encompassed in controversial arguments. The conflict in reporting comminution efficiencies has arisen from ill definition of the reference for the output energy. Surface area and surface energy have been widely used in the assessment of comminution efficiency. Review of the energy to produce new surface of quartz shows comminution efficiency ranging from about 0.1% to 1.0%, for various comminution methods, when expressed in terms of the surface free energy of quartz. However, in this paper, comminution efficiencies are based on the comparison of ball milling with single-particle comminution, either in terms of the energy to produce new surface or in terms of some size distribution parameter. If the energy to produce new surface by single-particle breakage is used as the basis for evaluating efficiency, then the efficiency of ball milling has a more realistic value of about 15% for the comminution of quartz and soda-lime glass. In a second approach, comminution efficiency is based on comparing the energy to produce some size distribution parameter of the product from ball milling with that by single-particle breakage. For both materials, dry ball milling efficiency was found to be in the range of 25%. (C) 2002 Elsevier Science B.V All rights reserved.