Catalyst Deactivation 1991 was an expanded version of earlier, highly successful symposia. The symposium featured invited and solicited papers including 4 plenary lectures, 78 oral presentations and 23 poster papers. Most of the papers are contained in this volume.The eight main topics emphasised at this most recent symposium were: deactivation mechanisms/phenomena (carbon deposition, poisoning, and sintering), methods (modeling and techniques), and important catalysts (hydrotreating, oxides, and zeolites). All of these areas were well represented as attested by the substantial number of papers contained in these proceedings. Four review papers based on the plenary lectures provide state-of-the-art perspectives on new thrusts in deactivation research and development.