Recent progress in periodic poling of magnesium-doped lithium niobate has made the material interesting for applications in second harmonic generation (SHG) of green light. The ternary phase diagram has been studied previously and a threshold composition identified above which the photorefractive effect is greatly suppressed. The typically produced 5 mol% MgO:LN material is grown from an off-congruent melt, so each grown crystal has a changing composition along its growth axis. Important performance parameters such as ease of electric field poling and phase-matching temperature are affected. In order to monitor crystal quality and reproducibility, accurate methods are needed to measure the composition of crystals above threshold. Chemical methods generally do not have sufficient sensitivity to resolve small variations. Measurements of the birefringent phase-matching temperature and UV edge location of samples from 21 grown crystals were analyzed and a correlation to the starting melt composition was established. Such measurements allow accurate prediction of the MgO content and to a somewhat lesser degree that of the Li/Nb ratio. (C) 2009 Elsevier B.V. All rights reserved.