Measurements of the refractive indices of single domain Sr₀.₆₁ Ba₀.₃₉Nb₂O₆ crystals are presented. A triangular prism cut from a melt-grown pure crystal was used in the study. Different lines of an Ar+ laser and a HeNe laser were used to illuminate the prism and measurements were carried out at room temperature and low light intensity to avoid heating of the sample. Refractive index values with an absolute accuracy better than ±0.002 were obtained using the minimum deviation method.