Surface-enhanced Raman scattering (SERS) spectra were obtained at a silver electrode in aqueous solutions at potentials from -0.2 to -0.6 V vs SCE and pH 1-7. The forms of 4-pyridinecarboxaldehyde that are adsorbed parallel those present in solution. In particular, the fraction of adsorbed species that is protonated at -0.2 V varies with pH in a manner almost identical with the fraction that is protonated in bulk solution. The protonated compound exists almost completely in the hydrated form (gem-diol) in the interface just as it does in solution. At a given pH, the fraction protonated on the surface increases as the potential is made more negative, a behavior attributed principally to enhanced adsorption of cationic species at negative potentials. In the presence of chloride, the fraction protonated decreases at the most negative potentials. This decrease is correlated with the desorption of chloride, suggesting that the pyridinium ion and chloride are adsorbed together. When the more strongly adsorbed bromide or iodide is used, the fraction protonated does not decrease at negative potentials.