Surface-enhanced Raman scattering (SERS) from the platinum electrode-acetonitrile interface in the presence of iodide, the lithium cation, water, and pyridine was analyzed as a function of applied potential. It was found that the typical Raman band of cyanide species by the dissociation of the solvent acetonitrile upon adsorption onto highly roughened platinum electrode surfaces was detectable for all of the systems that were studied. However, the onset potential of the dissociation reaction of acetontrile differed for the four systems. We assume that competitive adsorption may exist between each of the above four species and the solvent acetonitrile molecule, especially at the dissociation reaction-active sites of the Pt surface. This competitive adsorption therefore significantly inhibits the decomposition reaction of acetonitrile. The interactions of these adsorbates with Pt are assumed to weaken in the sequence pyridine > I- > water approximate to Li+ on the basis of the observations of different negative shifts of the onset potential of acetonitrile decomposition. For the system with water or iodide, double-band character for the CN band was also detected. It is assumed to be due to the existence of two types of adsorbed ion pairs at the Pt surface: CN-...CH3CN and CN-...Li+/Na+.