The optical properties of ultrathin Au films on silicon have been studied in the infrared over a wide frequency range from 200 to $\mathrm{10,000}{\mathrm{cm}}^{-1}$ . Thick films show a Drude behavior; i.e., with increasing frequency the transmission increases; for films below the percolation threshold at about $5\mathrm{nm}$ a negative slope for the frequency-dependent transmission is observed. When the thickness is further reduced, between 1 and $3\mathrm{nm}$ an anomaly occurs: the relative transmission reaches maximum values above 100% compared with the bare substrate, indicating an antireflection coating of nanometer thickness for light of $5\mathrm{\mu }\mathrm{m}$ wavelength. This anomaly can be explained in the framework of effective-medium theories.