The mechanism of catalyst attrition in cyclones has been investigated both theoretically and experimentally for various types of catalysts. A model is suggested that considers cyclone attrition under steady-state conditions as a comminution process. It considers the efficiency of such a process by relating the surface energy created by comminution to the kinetic energy, which has been spent to create this surface area. The attrition rate, defined as the mass of attrition-produced fines per unit mass entering the cyclone, is found to be proportional to the surface mean diameter of the material and to the square of the cyclone inlet velocity, and inversely proportional to the square root of the solids-to-gas loading ratio.