It is experimentally shown that the electrooptic properties of the ferroelectric phase of the new tungsten-bronze compositions can be treated as a biased quadratic electrooptic effect gij. This is done by directly measuring the electrooptic behavior above the transition temperature in KSr2Nb5O15. A term linear in electric field is not found. Thus, the crystal has a center of inversion. We find g38 — (ni/n3)3g13 = 0.10 m4/C2. This value is very similar to that obtained for a number of compounds containing NbO6 and similar octahedra. This value is also in good agreement with the effective room-temperature linear electrooptic coefficient rc, if it is assumed that rc can be considered to arise from a biased quadratic effect. Then we use these considerations systematically on other tungsten-bronze ferroelectrics, KSr2Nb5O15, NaBa2Nb5O15, Sr0.25Ba0.75Nb2O6, and Sr0.5Ba0.5Nb2O6. We calculate g33 and g13 separately and also find g33 — (n1/n1)3g13 = 0.10 — 0.11 m4/C2for all of these materials. Thus all these ferroelectrics give similar values to those found elsewhere for oxygen octahedra compounds. We also calculate values for the spontaneous polarization or the induced change in the indices of refraction. Copyright © 1968 by The Institute of Electrical and Electronics Engineers, Inc.