The longitudinal electro‐optic effect in ferroelectric tetragonal barium strontium niobate has been investigated in thin 45° plates of compositionx= 0.3. The half‐wave voltage for the linear electro‐optic effect at room temperature is approximately 1 kV. The half‐wave voltage decreases as the temperature approaches the transition and reaches a minimum value of about 100 V at 55 °C. The linear effect is observed to become quadratic near the transition and to remain quadratic up to the limit of observation (90 °C). Around the transition temperature the quadratic effect has a low half‐wave voltage, and is a slowly varying function of temperature. The high half‐wave voltage for the room‐temperature linear electro‐optic effect is attributed to the occurrence of a strongly nonlinear polarization characteristic. The quadratic effect is thought to occur when the material is in a depoled ferroelectric state or in a degenerate4¯2mphase.