Weathering and internal dissolution processes in mining waste materials may mobilize elevated levels of arsenic (As), contaminating ground and surface waters. Treating the polluted waters with iron oxyhydroxides is an established remediation method. By contrast, little knowledge is available to stabilize As in source materials by treating it with Fe precipitates and, on this way, to prevent the generation of polluted waters. In the present work the efficiency of Fe(II) treatment on As immobilization in a tailings material (TM) was studied with regard to the Fe:As molar ratio, the influence of CaCO3 amendment, and the As desorption at continued intensive leaching of Fe-treated TM. Fe precipitates were created by aerobic treatment of TM with Fe(II)sulfate at several Fe:As molar ratios with or without adding CaCO3, followed by aging the Fe-treated TM. The As retention in the treated tailings was studied by 4-fold elution with water, and the As desorption kinetics was examined by suspension leaching in laboratory microcosms over 3 weeks. Fe(II) treatment of TM reduced the water-extractable total As to <10 microg/L as the Fe:As molar ratio increased from 0 to 8. The water-soluble As of Fe-treated tailings could be reduced to 10-30 microg/L also under conditions of intensive leaching. Stabilizing the pH with CaCO3 resulted in consistently higher As release. The As desorption data followed the first-order kinetics in the early time stages of the desorption whereas at longer times the parabolic diffusion model was valid.