This proceedings contains papers from the IFAC Symposium on On-line Fault Detection and Supervision in the Chemical Process Industries (CHEMFAS-4), held in Jejudo Island, Korea, 7-8 June 2001. The proceedings includes theoretical contributions, as well as a wide range of industrial applications in process fault diagnosis, monitoring, and advanced supervision. The papers are organized around the following themes: fault detection and diagnosis, statistical and trend analysis, methodologies, sensor location and data reconciliation and applications. The driving forces for on-line fault detection and improved supervision of process operation include human safety, environmental safeguards, and equipment protection, as well as economic considerations such as the improvement of product quality, increased production, and so on. These diverse incentives, together with the development and evaluation of novel methodologies for on-line process supervision and management, form the focus of the symposium and of the papers in this Proceedings. Altogether over 60 papers are presented, covering strategies including model-based and data-driven approaches, as well as knowledge-based systems, statistical techniques, and AI-based pattern recognition techniques. All the work presented is at the cutting edge of research in this dynamic field.