This report describes the factual results of a unified bioaccessibility protocol undertaken as a part of the Environmental Geochemistry and Health project of the Environment and Health Programme of the British Geological Survey. In 2005 the Bioaccessibility Research Group of Europe (BARGE) undertook a joint decision to progress research in the field of in vitro bioaccessibility method development for the harmonisation of the use of bioaccessibility in human health risk assessment of contaminant soils in Europe. The progression took the form of combining their collective efforts to evaluate one in vitro method already in existence. The chosen method was that previously published by researchers at the Dutch Institute of Public Health, the RIVM, as this was considered to be most representative of the physiochemical conditions in the human gastrointestinal (GI) tract. Modifications to the RIVM methodology to ensure adequate conservatism, and that the in vitro test was robust and applicable to the local geological conditions in a range of different countries, were considered and tested prior to the preparation of a standard operating procedure (SOP) and detailed at the start of the full evaluation exercise. Evaluation of the unified bioaccessibility method (UBM) was undertaken by means of an international inter-laboratory comparison exercise. The BARGE members with affiliated laboratories who participated in the exercise were; the Dutch Institute of Public Health (RIVM), DHI soil and water (Denmark), INERIS (France), the University of Ghent (Belgium), Ohio State University (USA), the Royal Military College of Canada (RMC) and the British Geological Survey (BGS). The BGS undertook the role, in the evaluation exercise, as the lead laboratory, and the subsequent analytical requirement for the measurement of the bioaccessible element content in each extract was undertaken by the BGS Analytical Geochemistry Laboratories (AGL). The inter-laboratory study used soils and related materials with associated in vivo swine data, donated by Professor Nick Basta of the School of Environment Natural Resources at Ohio State University and by Professor Stan Casteel of the College of Veterinary Medicine at the University of Missouri. The materials under investigation include slag materials, soils, river sediments and house dusts containing the primary contaminants of concern, arsenic (As), cadmium (Cd) and lead (Pb). Main analytical performance characteristics of the bioaccessibility measurement (repeatability and reproducibility) were determined in the collaborative study using the procedure described in the ISO standard 5725-2.