The oxidative underpotential deposition of sulfur on Ag(111) from alkaline solutions of Na 2 S was investigated by in situ scanning tunneling microscopy (STM), cyclic voltammetry, and chronocoulometry. Proceeding toward more positive potentials, the cyclic voltammetric curve shows three partially overlapping peaks A-C and an isolated and more acute peak D. The STM images of the overlayer of adsorbed sulfur over the potential region between peaks C and D reveal a ( 3 × 3)R30°structure; those at potentials positive to peak D a ( 7 × 7)R19°structure: each lattice site of the latter structure is occupied by a triplet of sulfur atoms. The fractional coverage, 1 / 3 , for the ( 3 × 3)R30°structure is in perfect agreement with the maximum surface concentration, Γ max ) 7.7 × 10 -10 mol cm -2 , obtained from a thermodynamic analysis of the chronocoulometric charge vs potential curves; 2FΓ max is about 10% larger than the charge associated with the combination of peaks A-C. On the other hand, the 2FΓ value corresponding to the fractional coverage, 3 / 7 , for the ( 7 × 7)R19°structure agrees satisfactorily with the charge associated with the sum of peaks A-D, thus suggesting a total electron transfer from sulfide ions to the metal over the range of stability of the latter structure.