Single mode planar and channel waveguides have been produced in Sr0.6Ba0.4Nb2O6(tungsten bronze structure) by sulfur diffusion in a sealed ampule, followed by oxidation in an open tube. Losses in channel waveguides were ∼15–20 dB/cm for TM polarization and ∼27–32 dB/cm for the TE polarization inz‐cut substrates. Electro‐optic modulation was observed after poling of the substrate. The experimentally determined value of the effective electro‐optic coefficient was slightly greater than reported earlier for bulk samples of SBN:60, and about 15 times greater than for LiNbO3. Based on measurements with the S35radioisotope, the average atomic sulfur concentration was estimated to be about 4×1017/cm3in the region extending from the surface to a depth of 2.5 &mgr;m, and a significant background concentration (∼5×1016/cm3) was present to depths in excess of 20 &mgr;m.