Combination of the dithiol N,N '-bis(2-mercaptoethyl)isophthalamide, abbreviated as BDTH2 and as 1, with excess H2SeO3 in aqueous acidic (pH approximate to 1) conditions resulted in precipitation of BDT(S-Se-S) (6), with a Se-77 NMR chemical shift of delta = 675 ppm, and oxidized BDT. When the reaction is conducted under basic conditions Se(IV) is reduced to red Se(0) and oxidized 1. No reaction takes place between 1 and selenate (Se(VI)) under acidic or basic conditions. Compound 6 is stable in air but decomposes to red Se(0) and the disulfide BDT(S-S) (9) with heating and in basic solutions. Mechanisms and energetics of the reactions leading to 6 in aqueous solution were unraveled by extensive calculations at the omega B97X-D/aug-cc-pVTZ-PP level of theory. NMR chemical shift calculations with the gauge-independent atomic orbital (GIAO) method for dimethyl sulfoxide as solvent confirm the generation of 6 (calculated delta value = 677 ppm). These results define the conditions and limitations of using 1 for the removal of selenite from wastewaters. Compound 6 is a rare example of a bidentate selenium dithiolate and provides insight into biological selenium toxicity.