We have studied the electronic structure and optical properties, in the photon energy range of 3.0–4.5 eV, of sputtered Co/Ag multilayers with a scanning ellipsometer and Kerr spectrometer. It is found for the multilayer series Co(1.2 nm)/Ag and Co/Ag with dCo/dAg=2/3 that the screened plasma frequency ωp* of the multilayers and the interband transition frequency ω0 for L3→L2′ (d band to Fermi surface near the L point) in the Ag layers shift towards lower energy with increasing Ag layer thickness or modulation wavelength. For the second series of samples, the Kerr rotation peak also shifts towards lower energy with increasing modulation wavelength. For a third series of Co/Ag(1.3 nm) multilayers, only ω0 moves towards lower energy with an increasing Co layer thickness. The variation of ω0 is not easily explained in terms of internal strains and changes of the lattice constants in the Ag layers. Further theoretical analysis is needed.