The widespread implementation of distributed control and plant information systems has provided a reliable platform for advanced control in the process industries. Control loop performance assessment techniques have been utilized for identifying process control opportunities, and in the design, commissioning and maintenance of feedback control strategies. Numerous case studies have appeared in the literature, illustrating their successful application to petrochemical, and pulp and paper unit operations [1, 2]. To date, however, the potential benefits have not been as well documented in mining and metallurgy. The purpose of this paper is to present an introduction to the theory and practice of controller performance assessment, and to illustrate its value as a loop diagnosis tool, in troubleshooting the pH control of acid leaching at Cominco Ltd., Trail Operations. These results were obtained while beta-testing ProcessDoc - PA at Trail. Matrikon Control Performance Monitor is a commercial software package developed by Matrikon Applications Inc. for controller performance assessment and multivariate statistical analysis of industrial processes.