The ultra high vacuum (UHV) SERS excitation profile has been taken for two different adsorbates, pyridine and CO, on two different kinds of silver surfaces prepared in situ. The curves all show a maximum and in the case of pyridine the position of the maxima of the different vibrational modes vary slightly. The optical reflectivity spectra of these systems have also been measured using an oscillating beam spectrometer which compares the difference in the reflected light at two adjacent areas with and without the adsorbed molecules. The peak position of the excitation profile depend on both the adsorbate and the preparation of the silver surface. The excitation profile is not easily correlated with the optical measurements. It is concluded that multipolar resonant electromagnetic modes that are highly localized near the silver surface play an important role in shaping the SERS excitation profile. © 1983 Elsevier Sequoia S.A.