Arsenic contamination originating from mining waste is a widespread environmental problem. A principal issue for the assessment of As mobility in the environment and the development of possible site management strategies is the elucidation of the forms in which arsenic is transported or immobilized under local site conditions. This paper presents the results of investigations at a tin mill tailings site in Germany that is a major source of contamination to regional surface waters. Samples of the tailings, seepage water and secondary precipitates were investigated using a range of speciation methods. Possible mobilization and precipitation processes are discussed. The main path of arsenic mobilization seems to be via reduction, because the dominant valence of arsenic in the seepage water, and also in water samples from a well situated in the tailings pond, is As(III). The precipitates from iron hydroxides in the seepage water adsorb the arsenic but do not form stable arsenic compounds.