Surface Raman spectroscopy, electrochemistry, and X-ray photoelectron spectroscopy have been used to study the effects of air exposure on the stability of self-assembled monolayers (SAMs) formed from alkanethiols on mechanically polished, smooth Ag and Au surfaces. Raman spectra exhibit oxidized sulfur modes after only hours of air exposure. X-ray photoelectron spectroscopy of the S 2p region provides additional evidence for sulfur oxidation. Cyclic voltammetry of Ru(NH3)63+ indicates that oxidized alkanethiol SAMs retain blocking characteristics toward electron transfer, even after exposure of the oxidized SAM-surface to a solubilizing solvent. Control experiments suggest ozone as the primary oxidant in ambient laboratory air which causes rapid oxidation of the thiolate moiety. These results have important ramifications for the general use of SAMs in many proposed applications.