We have measured the vibrational spectra of various coverages of CO adsorbed on a polycrystalline Pt electrode in aqueous Na2S04, K2S04, and KC104electrolytes at a potential of -0.350 V (SCE). Compared to spectra recorded in H2S04and HC104at +0.150 V, several changes are observed. The bridging band becomes easily resolvable, even at low CO coverages where its intensity in fact approaches that of the linear band. The frequency of the linear band at saturation coverage decreases from 2100 to 2090 to 2060 cm-1 when results measured in vacuum, acidic electrolyte, and neutral electrolyte, respectively, are compared. The isolated coverage frequency decreases from 2065 to 2030 to 1963 cm-1when the same comparison is made. These results are attributed to differences in the electric field at the metal surface, due to the combined effect of changes in electrode potential and the extent of specific adsorption of electrolyte cations.