The surface enhanced Raman scattering (SERS) phenomenon due to pyridine on silver and CO on silver in ultra-high vacuum (UHV) has been compared. The study was carried out for two different kinds of silver surfaces, island films and "cold" films on island films. We have reported earlier that it was necessary to warm the samples to about 70K in order to observe the full surface enhanced Raman spectrum when the pyridine was deposited on silver island films on sapphire substrates at liquid He temperatures. We call this the effect of a "low temperature anneal" and the present work indicates that this effect is more general than we originally thought. It has also been found that island films exhibit enhanced Raman scattering for pyridine but not for CO, although "cold" films enhance the Raman scattering of both CO and pyridine. This is the first clear indication that molecular specificity may be a part of the SERS phenomenon. The interpretation of these observations depends critically on our understanding of the SERS phenomenon. © 1982.