Partly oxidized Ag films deposited on glass substrates were found to exhibit significantly different reflectance properties for either s- or p-polarized incident light under conditions of total internal reflection. The measured reflectances depend strongly on angle of incidence and wavelength of light and an sensitive functions of the Ag and Ag2O layer thicknesses. For pure and slightly oxidized Ag films we observe the evolution of the well known surface plasmon resonance, whereas for Ag films with an increasing oxide layer a new resonance is observed in a-polarization. We account for all these effects by modeling the oxidized Ag film as a glass/Ag/Ag2O/air system and using the recently published optical constants for Ag2O.