In this work, the batch-reactor reduction of selenious acid (H2SeO3) species with sodium dithionite (Na2S2O4) from weakly acidic sulfate solutions containing 300 mg/L of selenium at 23 degrees C was studied. The results showed that, at an initial pH value of <1.7 and a dithionite stoichiometric excess of > 3, <0.5 mu g/L of selenium(IV) remained in solution after reduction. The reductive precipitation reaction started as soon as dithionite was added in the selenium-bearing solution and was completed within less than a minute. However, it was determined that the precipitate was not stable in the presence of the dithionite decomposition byproduct and partially redissolved after several hours. The reaction product, characterized using X-ray diffraction, scanning electron microscopy, and chemical analysis, was determined to be red amorphous selenium. The precipitate, in addition to elemental selenium, was determined to contain monoclinic sulfur that was apparently formed via a side reaction pathway involving the decomposition of dithionite.