Silver has useful surface-plasmon-resonance properties for many potential applications. However, chemical activity in silver nanoparticles exposed to laboratory air can make interpretation of optical scattering and extinction spectra problematic. We have measured the shift of the plasmon polariton wavelength of arrays of silver nanoparticles with increasing exposure to ambient laboratory air. The resonance peak wavelength shifts 65 nm in 36 h (1.8 nm/h). We show by scanning Auger spectroscopy that the shift is due to contamination from sulfur, most likely chemisorbed on the surface. The rate of corrosion product growth on the nanoparticles is estimated to be 3 nm per day, 7.5 times higher than that of bulk Ag under the same conditions.