Overlayers of chemisorbed sulfur, S-chem On polycrystalline Pt can be formed spontaneously by Pt immersion in aqueous Na2S solution and they entirely suppress the underpotential deposition of hydrogen, UPD Pi. Au,ner electron spectroscopy and core-level electron energy loss spectroscopy measurements indicate that S-chem is not present in an oxidized or reduced state; S-chem has a partial negative charge (well below unity) due to charge redistribution between the Pt substrate and the S-chem adsorbate. The layer of S-chem can be gradually removed from the Pt surface through its oxidative desorption under cyclic voltammetry conditions in clean H2SO4 electrolyte. The oxidative desorption was strongly affected by the temperature variation. The S-chem desorption under T-control conditions resulted in formation of submonolayers of S-chem whose coverage could be controlled to within 5% of a complete overlayer. The UPD Pi on Pt free of S-chem follows the Temkin adsorption isotherm, while in the presence of S-chem having theta(S) = 0.10, the process follows a Frumkin-type adsorption isotherm with the energy of later interactions being T-dependent. The undeipotential-deposited Pi, H-UPD, reveals enhanced surface mobility in the presence of S-chem than in its absence. The S-chem overlayer (theta(S) = 0.10) significantly decreases the Pt-H-UPD, surface bond energy. This behavior can be explained in terms of the "electronic blocking" effects which arise from direct electrostatic interaction between the adsorbates.