We determined the optical properties of α - and γ -phase Ce in the photon energy range from 60 meV to 2.5 eV using ellipsometry and grazing incidence reflectometry. We observe significant changes of the optical conductivity, the dynamical scattering rate, and the effective mass between α - and γ -cerium. The α -phase is characterized by a Fermi-liquid frequency-dependent scattering rate, and an effective mass of about 2 0 m e on an energy scale of about 0.2 eV. In γ − Ce the charge carriers have a large scattering rate in the far infrared and a carrier mass characteristic of 5 d band electrons. In addition, we observe a prominent absorption feature in α − Ce , which is absent in γ − Ce .