Operating conditions during the air classification process, of critical importance for many grinding operations, were tested in laboratory scale. For efficient and satisfactory operation, the most important parameters, such as air flow rate, concentration of material in the air, rotor speed, wear rate, classification capacity and efficiency, require optimization. This paper indicates the influence of operating parameters on the classification process and provides optimization examples carried out by practical testing. On-line particle size and concentration measurement from Malvern Insitec was used to investigate the flow conditions during testing. Direct on-line measurement was possible inside the classifier circuit, thanks to the "multi-scattering" algorithm system, designed for the on-line measurement of highly concentrated particles in flows ranging from static to supersonic. (C) 2002 Elsevier Science Ltd. All rights reserved.