The efficiency of As immobilization by treating a tailings material (TM) from an abandoned tin mill tailings pond with Fe oxyhydroxides and Al hydroxides was investigated with respect to the influence of the amount of sorption agents, the effects of pH buffering with CaCO₃, and the As desorption kinetics under continued intensive leaching of treated TM. Fe and Al precipitates were created by aerobic treatment of TM with Fe(II) or Al sulfate solutions with and without CaCO₃ amendment, followed by aging the treated TM. The As retention in the treated TM was studied by a water elution procedure and by suspension leaching in microcosms. Fe(II) treatment reduced the water-soluble total As from 340 to <10 μg/L as the Fe:As molar ratio increased from 0 to 8. The As released during suspension leaching of TM treated at the Fe:As molar ratio of 8 did not exceed 30 μg/L. Stabilizing the pH with CaCO₃ resulted in higher As release. The As desorption kinetic data were well fitted by the parabolic diffusion rate expression, suggesting that the desorption of As from Fe-treated TM was diffusion-controlled. TM treated with Al sulfate at pH 7 showed high As retention already at the Al:As molar ratio of 2. Buffering with CaCO₃ required an Al:As molar ratio of 5 to reach As concentrations <10 μg/L in the eluates. The results of this initial study demonstrate that As can be stabilized in source material via interaction with amorphous Fe or Al precipitates. The efficiency of As stabilization is primarily related to the amount of Fe or Al addition.