The baseline chemistry of groundwaters varies both spatially and with time due to many complex factors including climate, soil type, geology and residence time. The baseline is therefore represented by a range rather than a single concentration. The variations in baseline groundwater characteristics are presented for a variety of aquifers in Europe including different rock types (from carbonate to siliclastic) and ages (PreCambrian to Quaternary). Most aquifers have been affected to some degree by diffuse pollution and a set of guidelines has been developed to help in the determination of baseline.More than 80 components were measured in these aquifers. Statistical data and plots are useful for comparison between areas and are of strategic value since they include data for elements for which there are no existing guidelines, but which may still be harmful (e.g. Be, Se, Sb, Tl, U). This represents a comprehensive and strategic database of the baseline concentrations for most inorganic substances and a reference to quantify pollution by such substances.There is considerable overlap between the different aquifers for most solutes regardless of main aquifer type. The main hydrochemical characteristics in aquifers are determined, not specifi cally by rock type, but by a range of hydrochemical processes such as mineral dissolution, redox reactions, ion-exchange reactions, mixing, adsorption-desorption and rainfall inputs.