The use of radars to study the geophysical features in a polar environment requires reliable information about the relative complex dielectric permittivity, ϵ=ϵ'+iϵ″, of ice at microwave and millimeter-wave frequencies. To address this requirement, ϵ of ice at millimeter wavelengths was obtained using an interference technique. The interference pattern was produced by measuring the transmittance through bubble-free ice slabs at normal incident angle as the frequency was swept from 75 to 110 GHz. From the resulting interference patterns, ϵ' was determined to be 3.17¿0.05 and virtually independent of frequency. Frequency-dependent absorption loss was observed such that ϵ″ increased from approximately 2.4¿0.2¿10-3 to 4.0¿0.2¿10-3 over the swept frequencies.