Phase transitions in

linear birefringence (LB) of 0.61Ba0.39Nb₂O₆:Ce³⁺(SBN61:Ce) has been measured as a function of temperature within the range of78 \≤ T \≤ 850. Large tails have been observed above ferroelectric phase transition temperatures T_ = 350, 328, 320 291 K for the concentrations x = 0, 0.0066, 0.0113 and 0.0207, . Within an Ornstein-Zernike analysis the critical exponents γ, ν and β are determined. It suggests that pure SBN61 belongs to the 3D Ising class. Doping with ³⁺ ions, which seem to act as fields, enhances the relaxor properties. The critical exponents γ ν of SBN61:Ce shift against those of the three-dimensional -field Ising model.