We report the first in situ observation of the effect of CO coadsorption on the SERS spectra of pyridine adsorbed on silver in a UHV system. This is of particular interest because SERS of pyridine and water in an electrochemical system is strongly affected by the concentration of chloride ions—a phenomenon which seems to manifest the chemical aspect of SERS. Unexpected but significant changes are seen in the SERS spectra of pyridine when exposed to CO gas. The effect depends on the type of silver surface and involves shifts in the peak positions of some of the vibrational modes as well as changes in their relative intensities. In the case reported here we do not see the SERS bands around 2100 cm−1 which are normally attributed to the CO vibrations. A possible interpretation is that surface complexes are formed between the pyridine and CO molecules which are chemisorbed to ’’active sites’’ on the silver surface.