For noble-metal particles embedded in a SiO 2 matrix, the s-d hybridization has been studied by investigating the damping of the optical plasma-resonance absorption of noble-metal island films in the photon-energy region from 1.5 to 6.5 eV. The s-d hybridization enhancement is proposed to explain qualitatively the large damping at small size. This enhancement is based on the d band broadening due to lattice contraction. The effect of s-d hybridization enhancement on the damping is conspicuous below about 30 Å in diameter.