More than 10 years after the "First European Symposium on Organic Micropollutants in the Aquatic Environment", the "Sixth Symposium" was held in Lisbon (Portugal) from 22 to 24 May 1990. The Symposium was organised within the framework of the Concerted Action COST 641 * which is included in the Fourth R&D Programme on the Environment of the Commission of the European Communities expiring at the end of 1990. After restructuring the Concerted Action in 1984, particular attention has been devoted to fate and transformation of organic micropollutants in the aquatic environment. Therefore, a major aim of the Symposium was to review current studies and progress in these areas, besides more general aspects related to analytical methodologies and behaviour of pollutants during water treatment processes. This volume contains the plenary papers presented in the following sessions at the Symposium: - Multidisciplinary studies - Partitioning of organic micropollutants in the aquatic environment - Novel analytical techniques in environmental chemistry - Monitoring micropollutant - Degradation of organic micropollutants during field conditions - Reclamation of polluted ground water - Chemical and photochemical oxidation - Future of environmental chemistry. In addition, extended versions of posters are included, covering the four topics which are dealt within the four Working Parties forming the structure of the Concerted Action: 1. Analytical methodologies 2. Transport and distribution 3. Transformation reactions 4. Water treatment. We believe that the Proceedings give a good overview of current activities in these fields of research in Europe.