Interaction of aqueous Se(IV) with pyrite was investigated using persistently stirred batch reactors under O2-free conditions at pH ranging from 4.5 to 6.6. Thermodynamic calculations, an increase in pH during the experiments, and spectroscopic observation indicate that the reaction of aqueous Se(IV) by pyrite results in the formation of Se(0) and the release of Fe(II). The aqueous Se(IV) reduction rate follows a pseudofirst order. A ferrous iron mediated electron transfer mechanism is proposed to operate during the reduction of aqueous Se(IV) by pyrite. pH and iron concentration affect significantly on Se(IV) reaction rate and reaction product.