The present paper is a partial theoretical approach to the comminution process. A general theory of comminution should consist of two parts, one that deals with the energy required to break mineral particles and another that examines how this energy is distributed to the particles generated after breakage. The present approach deals with the second part that examines how the energy invested for comminution is distributed to the mill product. It uses the generally accepted concept, which assumes that the useful part of comminution energy is consumed to create new surfaces and finds the relationship between a characteristic particle size of the mill product and the energy consumed for grinding. The paper introduces the concept of potential energy and provides the means to give a value to the energy state of a material produced by a specific type of equipment. The energy efficiency is also taken into consideration and is used to calculate the energy actually invested for comminution. The main conclusion is that the specific surface energy is a physical property of materials and can be used as a universal index characterizing their grindability, regardless of the mill type or the mill efficiency. The physical dimensions of this index are energy per unit surface area (J/m(2)) compared to energy per unit volume or unit mass, which are the dimensions of the indices proposed so far. (c) 2006 Elsevier Ltd. All rights reserved.