The BGS Geochemical Baseline Survey of the Environment (G-BASE) was the national strategic geochemical mapping programme in Great Britain. The project set out to establish the chemistry of the surface environment by the collection and analysis of stream sediment, stream water and soil samples. Beginning in the late 1960s in northern Scotland and moving southwards across the country, the primary focus was mineral exploration, however, the project quickly developed to address important environmental concerns. The final G-BASE samples were collected in southern England in 2014. The outputs from the G-BASE project provide an invaluable, systematic baseline of geochemical information for Great Britain, serving as a marker of the state of the environment against which to measure future change. Stream sediments were the primary sample medium for G-BASE, with an average density of one site per 1.5 km square. The drainage sampling sites cover most of Great Britain. Sediment was collected from the centre of the stream and sieved through two sieves (2 mm and 150 µm) to obtain a fine grain-size fraction of <150 microns. Analytical data (by a variety of analytical methods, including XRF and direct-reading optical emission spectrometry (DR-OES) for the <150 micron fraction of stream sediment samples are available for some or all of the following elements: Ag, As, Ba, Bi, Br, Ca, Cd, Ce, Co, Cr, Cs, Cu, Fe, Ga, Ge, Hf, I, K, La, Mg, Mn, Mo, Nb, Ni, P, Pb, Rb, Sb, Sc, Se, Se, Sn, Sr, Ta, Te, Th, Ti, Tl, U, V, W, Y, Zn and Zr. Some stream sediment samples were originally collected as part of the Mineral Reconnaissance Programme (MRP), and later re-analysed for G-BASE. These samples may have been assigned a new sample number, but will plot at the same site. For more information about accessing these samples and their analytical results, contact BGS Enquiries (enquries@bgs.ac.uk ).