While the optical properties of thin metal films are well understood in the visible and near-infrared range, little has been done in the midinfrared and far-infrared region. Here we investigate ultrathin gold films prepared on $\text{Si}(111)(7\ifmmode\times\else\texttimes\fi{}7)$ in UHV by measuring in the frequency range between 500 and $7000\text{ }{\text{cm}}^{\ensuremath{-}1}$ and for temperatures between 300 and 5 K. The nominal thickness of the gold layers ranges between one monolayer and 9 nm. The frequency and temperature dependences of the thicker films can be well described by the Drude model of a metal when taking into account classical size effects due to surface scattering. The films below the percolation threshold exhibit a nonmetallic behavior; the reflection increases with frequency and decreases with temperature. The frequency dependence can partly be described by a generalized Drude model. The temperature dependence does not follow a simple activation process. For monolayers we observe a transition between surface states around $1100\text{ }{\text{cm}}^{\ensuremath{-}1}$.