Lithiun niobate crystals have been grown from a range of selected melt compositions. Index of refraction measurements on the congruent composition at 6328 Å and 20° C are reported. Variation in index with composition is examined with respect to the congruent composition, used as the standard I0 substrate, to assess its compositional uniformity. Index measurements for congruent lithium tantalate are also reported. Material and composition related properties which can effect I0 substrate behavior are discussed.