Electron spectroscopic and diffraction results obtained in ultra-high-vacuum, combined with cyclic voltammetric data, are reported for sulfur adlayers deposited from aqueous sulfide and bisulfide media on Pt(lll). The highest coverage obtained by Auger electron spectroscopy, 0.94 +/- 0.05 monolayer, is very close to the coverage obtained from coulometry, and is associated with a (1x1) surface phase. This coverage is much higher than that obtained in previous electrochemical studies but is the same as found by other investigators using S-2 beam dosing in vacuum. The near complete sulfur monolayer is characterized by a rapid and incomplete oxidation in a narrow potential range near 0.70 V vs a Ag/AgCl reference. Neither full sulfur monolayer coverage nor a sharp voltammetric transition could be obtained when traces of oxygen were present in the electrochemical cell. Oxidation of the (Ix I) adlayer (at approximate to 1 monolayer) gave-rise to a previously unreported (2x2) structure, at 1/2 monolayer. Further voltammetric stripping resulted in two more adlattices : (root 3 x root 3)R30 degrees at 1/3 monolayer and p(2x2) at 1/4 monolayer, as reported in previous gasphase studies. The selective stripping procedure provides unique electrochemical control at room temperature of surface structure and coverage, without any change in the long-range surface order of the substrate. When dosing was carried out from bisulfide solution, a (root 3 x root 7) phase at 3/5 monolayer was formed, which once again was not reported previously. The results of the core-level electron energy loss spectroscopy studies suggest that sulfur adatoms retain some of the negative charge and that this charge plays a major role in controlling hydrogen adsorption coverage in the presence of coadsorbed sulfur on Pt(111).