The formation and reactivity of sulfur adlayers on single-crystal Ag electrodes ((111), (110), and (100) orientations) in aqueous solutions (pH = 13) containing HS- are reported. Oxidative adsorption of HS- (Ag + HS- --> AgSH + e(-)) occurs on all three low-index surfaces at potentials ranging between -0.5 and -0.7 V of the thermodynamic value for bulk Ag2S formation. Voltammetric and electrochemical quartz crystal measurements demonstrate that the resulting AgSH adlayer undergoes a second one-electron oxidation (AgSH + Ag + OH- --> Ag2S + H2O + e(-)) at the (111) and(110) surfaces prior to bulk Ag2S formation, yielding an underpotential deposited Ag2S adlayer (surface coverages (S/Ag) : theta(Ag(111)) = 0.46 +/- 0.02 and theta(Ag(110)) = 0.54 +/- 0.03). In contrast, the AgSH adlayer on Ag(100) is chemically inert prior to bulk Ag2S formation. Structural models indicate that the formation of a nearly stoichiometric Ag2S adlayer (i.e., theta similar to 0.5) is feasible on the (111) and (110) surfaces without significant reconstruction of the outermost atomic layers of the substrate, but not on the (100) surface. The results suggest that the formation of a Ag2S monolayer is allowed only when the number density of S and Ag atoms at the interface is nearly coincident with the reaction stoichiometry.