The structure of the layers formed on a silver electrode by the adsorption of pyridine, Cl−, CN− and SO42− is discussed. A comparison of the amount of the species adsorbed with their Raman spectra shows that there is no simple relation between the species concentration and the enhancement of the Raman intensity. Furthermore, the inhomogeneity of the layers is shown by Raman microscopy observations. These results lead us to propose the formation of a new species stabilized in the colloidal silver layer. The Raman enhancement could be explained by the existence of a resonance effect.