The kinetic of the oxidation of trivalent arsenic was investigated in synthetic as well as in natural samples of a tin mill seepage water. The influence of ferric ions and solid MnO(2) on the process was studied. To determine the time dependence of the concentrations of the arsenic species, a series of samples were taken sequentially and analysed by coupling of ionchromatographic separation and ICP-MS detection. To investigate the naturally occurring oxidation reaction, original seepage water samples were filtered and spiked with As(III) (1 mg l(-1)) and Fe(II) (10 mg l(-1)) and shaken providing intensive contact with the air. Additional synthetic samples buffered with carbonate were used in similar experiments to simplify the system. The reaction was incomplete in the presence of excess of iron for both types of samples. The oxidation of As(III) was complete within 8 h in the presence of MnO(2) (10 mg), but there was a difference in oxidation rates between the natural and the synthetic samples. The results are discussed with respect to the redox potentials and equilibrium constants.