We present cyclic voltammetry studies of the under- and overpotential deposition of copper on a Pt(111) electrode in aqueous 0.50 M sulfuric acid solution in the presence and in the absence of adsorbed layers of 2- and 4-mercaptopyridine (2-MP, 4-MP). In general, the presence of adsorbed layers has been found to inhibit electrodeposition processes, suggesting very strong interactions between the adsorbates and the Pt(111) surface. 2-MP originates a higher degree of inhibition, which is ascribed to a stronger interaction probably due to the formation of a surface chelate via bonding through both N and S atoms. Studies have also been made of the reaction of 2-MP and 4-MP in solution with a Pt(111) electrode covered by an electrodeposited copper monolayer. The latter is shown to be partially or completely displaced (oxidized), depending on the electrode potential, by 2-MP and 4-MP. The reaction of the copper monolayer with 2-MP and 4-MP gives rise to an adsorbed intermediate, and a possible mechanistic pathway for this process is proposed.