The aim of the EU Acute Exposure project, ACUTEX, is to develop a methodology for establishing European Acute Exposure Threshold Levels, EU AETLs, for toxic substances in relation to harm to people by inhalation. The development of AETLs is initially in the context of the risks of major accidents from chemical sites and in particular their regulation through the EU 'Seveso II' Directive. It is intended that AETLs, can be used within Member States, where appropriate, to inform decisions on land-use planning and emergency planning. AETLs will not have a regulatory status. This paper describes: the selection of 21 preliminary substances to use as case studies in the development and testing of the AETL's methodology; and the development of a prioritisation methodology to inform initial substance selection for a possible further AETLs program. The work was based on consultation with experts drawn from EU major stakeholder groups. It included a Validation Exercise working with three Member States, which account for between approximately 40% and 50% of all EU Seveso II sites. From this Validation Exercise we infer that, if these three Member States are representative in terms of numbers of priority substances, then the number of EU higher priority substances for further AETLs development is unlikely to be much in excess of 50. (c) 2005 Elsevier B.V. All rights reserved.