Technetium-99 (Tc-99) is important to the nuclear fuel cycle as a long-lived radionuclide produced in similar to 6% fission yield from U-235 or Pu-239. In its most common chemical form, namely, pertechnetate ((TcO4)-Tc-99 ), it is environmentally mobile. In situ hydrogen sulfide reduction of pertechnetate has been proposed as a potential method to immobilize environmental (TcO4)-Tc-99 that has entered the environment. Reactions of (TcO4)-Tc-99 with sulfide in solution result in the precipitation of Tc2S7 except when olefinic acids, specifically fumaric or maleic acid, are present; a water-soluble Tc-99 species forms. NMR (H-1, C-13, and D-2 methods) and X-ray absorption spectroscopy [XAS; near-edge (XANES) and extended fine structure (EXAFS)] studies indicate that sulfide adds across the olefinic bond to generate mercaptosuccinic acid (H(3)MSA) and/or dimercaptosuccinic acid (H4DMSA), which then chelate(s) the Tc-99 to form [(TcO)-Tc-99(MSA)(2)](3), [(TcO)-Tc-99(DMSA)(2)](5), or potentially [(TcO)-Tc-99(MSA)(DMSA)](4). 2D NMR methods allowed identification of the products by comparison to Tc-99 and nonradioactive rhenium standards. The rhenium standards allowed further identification by electrospray ionization mass spectrometry. (TcO4)-Tc-99 is essential to the reaction because no sulfide addition occurs in its absence, as determined by NMR. Computational studies were performed to investigate the structures and stabilities of the potential products. Because olefinic acid is a component of the naturally occurring humic and fulvic acids found in soils and groundwater, the viability of in situ hydrogen sulfide reduction of environmental (TcO4)-Tc-99 as an immobilization method is evaluated.