Magnesium films have been traditionally used for UV filters. However, scarce information has been reported on its optical properties [1-3] and the problems it presents in the production of the films. Being a Drude material, magnesium is a suitable material for studying the photon-surface plasmon interaction by attenuated total reflectance, but the dielectric function is poorly known for the IR-visible region. In this paper we present our values for the optical properties of a Mg thin film obtained from ellipsometric and absolute reflectance measurements. The dielectric function is estimated using Drude modeling, extending the previously reported data into the infrared.