The effect of water on the infrared spectrum of adsorbed CO has been studied for a Pt/K L-zeolite sample. The red shift of the CO spectra and the intensity transfers to lower frequency bands observed with increasing uptake of water are attributed to an increased electron density of the metal particles. Adsorption of CO under "wet" conditions results in an increase in the ratio of integrated intensities of bridged to linear CO species, consistent with an increase in the electron density on the metal particles. It is proposed that the water molecules associated with the zeolite cations modify the metal-support interaction via the zeolite lattice.