The paper presents the advanced control systems installed by the authors in two gold ore grinding circuits in South Africa, with three lines at Anglogold Ashanti Mponeng mine and six lines at Anglogold Ashanti Kopanang mine. At both plants the advanced control system is based on the OCS(C) software, and combines the flexibility of an expert system with fuzzy logic and the power of an on-line dynamic phenomenological model used as an advanced soft sensor of the whole process. The history and implementation methodology is described, and the results-increases in throughput and gold recovery-are analysed. The improved control of particle size makes it possible to maximise gold recovery. This is discussed and illustrated. (C) 2005 Elsevier Ltd. All rights reserved.