The optical properties of Dow Corning‐704 and ‐705 diffusion‐pump oils have been measured from 2→10.6 eV using a closed‐cell technique. The data are interpreted in terms of molecular excitations of &pgr;, &sgr;, and nonbonding electrons in the complex pump oil molecules and in terms of collective oscillations of electrons in the liquids. The optical constants of Dow Corning‐200, another silicone oil, and liquid benzene from 2→10.6 eV are also presented for use in the interpretation of the pump oil data.