Tungsten bronze ferroelectric crystals SrxBa1−xNb2O6and Ba2−xSrxK1−yNayNb5O15have been investigated for potential electro‐optic uses at millimeter wave carrier frequencies between 55 and 110 GHz. Indices of absorption and refraction at temperatures between 20 and 300 K were derived from measurements of transmittance spectra containing Fabry–Perot fringes produced by crystal surface reflections. The anisotropies observed in both indices at these frequencies are huge compared with those at optical frequencies. Absorption in both materials decreases markedly upon cooling, but only for incident waves polarized along the crystal polar axis.