Reversible, oxidative adsorption of hydrosulfide (HS-) and ethanethiolate (CH3CH2S-) on highly-ordered, Ag(111) electrodes in aqueous 0.5 M NaOH solutions is reported. Oxidation of HS- and CH3CH2S- results in adlayer formation, as determined by voltammetry, coulometry, and in-situ electrochemical quartz-crystal measurements. The voltammetric response of Ag(111) in HS- solutions displays three characteristic waves. Two waves correspond to the reversible 1-e- oxidative adsorption of HS-, resulting in the formation of a Ag-SH adlayer. The third wave corresponds to the kinetically-slow 1-e- oxidation of the Ag-SH adlayer, yielding an underpotential deposited monolayer of Ag2S. The voltammetric response observed in CH3CH2S- solutions is qualitatively similar to that of HS-, displaying two well-resolved waves corresponding to oxidative adsorption. The observation of two voltammetric waves is suggestive of a mechanism of CH3CH2S- adlayer formation involving at least two distinct structural phases. However, unlike the situation for HS-, the resulting Ag-SCH2CH3 adlayer is unable to undergo a second oxidative transition. The free energies of adsorption of HS- and CH3CH2S- on Ag(111) are measured to be ca -23 kcal/mol.