Abstract The objective of the GREAT-ER project is to develop and validate a powerful and accurate aquatic chemical exposure prediction tool for use within the EU environmental risk assessment schemes. Current techniques to estimate regional PECs use a generic multimedia `unit world' approach and do not account for spatial and temporal variability in landscape characteristics, river flows and/or chemical emissions. Hence, the results are merely applicable on a generic screening level since these models do not offer a realistic prediction of actual steady-state background concentrations. In addition, the default EU generic regional environment (EU Technical Guidance Documents, 1996) only allows treatment for 70% of the waste water mass loading, leaving 30% of mass loading to this generic region untreated. A new database, model and software system will be developed to calculate the distribution of predicted environmental concentrations (PEC), both in space and time, of down the drain chemicals in European surface waters on a river and catchment area level. Data on dissolved oxygen, biological oxygen demand and ammonia will also be used to assess water quality and to provide data for calibration and validation. The system will use Geographical Information Systems (GIS) for data storage and visualization, combined with simple mathematical models for prediction of chemical fate. Hydrological databases and models will be used to determine flow and dilution data. This refined exposure assessment tool should greatly enhance the accuracy of current local and regional exposure estimation methods. The new exposure assessment methodology will integrate specific environmental information and be worked out in a geographically-referenced framework, ultimately on a pan-European scale. This research project is carried out on behalf of ECETOC, and sponsored by the Environmental Risk Assessment Steering Committee (ERASM) of the Association Internationale de la Savonnerie, de la Detergence et des Produits d'Entretien (A.I.S.E.) and the Comite Europeen de Agents de Surface et Intermediares Organiques (CESIO) in cooperation with the UK Environment Agency.