Tetramethylammonium sulfide [(CH3)4N]2S dissociates in water yielding tetramethylammonium cation, which does not hydrolyze, and sulfide anion (S2-), which undergoes hydrolysis reactions forming HS- and H2S. The reaction of these three sources of S with the surface of SiGe(lOO) 25% was investigated by X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD), and a speciation model as a function of pH. Lowering the pH of dilute aqueous TMAS increased the H2S concentration and the S coverage on the surface increased in tandem. Covalent Ge-S bonds formed, and the highest S/elemental Ge molar ratio was 0.66 at pH 2-3. The surface termination was mixed, that is H-SiGe-SH. H2S desorbed in a state with a peak temperature at about 550 K and GeS in a peak at about 600 K. The Ge atoms on the SiGe surface were preferentially oxidized in aqueous TMAS based on the O/Si and Ge13+/Ge molar ratios calculated from XPS data compared to control surfaces.