Gold films with a thickness (d = 3 to 10 nm) around the percolation threshold are investigated in a broad spectral range. Their optical conductivity is dominated by two contributions: a Drude-component present above a critical thickness and a plasmon in the near-infrared region, which shifts down in frequency as d decreases. The interplay of both components leads to a maximum in the dielectric constant epsilon(1) (d) observed right below the insulator-to-metal transition. For even lower coverage, we find an additional dielectric anomaly where the reflectivity for Au on Si/SiO27 completely vanishes in the infrared region. (C) 2010 Elsevier B.V. All rights reserved.