Differential-thermal analysis indicates that phase instability occurs in the majority of melt-solutions of the BaO-B2O3-Na2O system, from which crystals of barium metaborate (BaB2O4, BBO) can be grown. Experiments show that the arising inhomogeneity of the crystallization medium accompanied by liquation phenomenon might result in faulty seeding and growth of BBO crystals, difficulties in controlled shaping, trapping of abundant inclusions, and other negative effects. The composition of the widely used solvent in growing BBO crystals ("eutectic" between the composition of BaB2O4 and BaB2O4 (.) Na2O) is unstable and tends to polyvariant transformations and, hence, is far from optimum. The compounds Na4BaB4O9 and NaBO2 might be the components that form pseudo-binary systems with BBO relative to stable melt-solutions promising for further technological processing. (C) 2002 Elsevier Science B.V. All rights reserved.