The optical properties of Dow Corning‐704 and ‐705 diffusion‐pump oils have been measured from 4→24.8 eV using an open‐dish technique. These are the first liquids for which optical constants have been obtained above 11.8 eV. In the region above 10.6 eV, the real part of the dielectric constant is structureless. A collective oscillation involving &pgr; and &sgr; electrons is seen in the region between 21→25 eV. Sum rule calculations indicate approximately 130 as the effective number of electrons per molecule participating in absorption processes up to 25 eV.