The temperature variations of the index of refraction and of the birefringence were studied in nominally pure SrTiO₃ and Sr_1-xBa_xTiO₃ single crystals (x=0.025, 0.07 and 0.15), In Sr_1-xBa_xTiO₃ (x=0.15 and 0.07) phase transitions from the paraelectric phase to a ferroelectric one were detected. A contribution of the spontaneous electrooptical effect to the temperature variation of the refractive index was obtained. This allowed us to evaluate the spontaneous polarisation and its temperature variation. For x=0.025 the transition at T ≈ 28 K was explained by a polar state with glass-like behavior.