The subsolidus phase relations of the ternary system, Li2O-MgO-B2O are investigated by the powder X-ray diffraction method. The system can be divided into 16 three-phase regions and three two-phase regions. There exist 11 binary compounds and two ternary compounds. A new compound with the composition close to Li2 45Mg0.3BO3.025 is identified. It has a hexagonal structure with cell parameters a=4.9408(6) Angstrom, c=17.690(4) Angstrom. (C) 2002 Elsevier Science B.V. All rights reserved.