A transverse quadratic electro‐optic effect has been observed in a wide range of niobate transparent glass—ceramic systems. The electro‐optic constant has been determined as a function of composition, temperature, and wavelength. A semiquantitative measurement of the frequency response has also been made. It has been found that the electro‐optic constant correlates reasonably well with the square of the static dielectric constant. Remanent polarization has been induced under the influence of a dc field. The resulting linear electro‐optic effect has been measured in both transverse and longitudinal modes.