Phase equilibria in the Li2O-B2O3 system were studied in the composition range from 48 to 83 mol % B2O3. The liquidus temperatures were determined with a high accuracy for 25 compositions, and the temperatures of invariant equilibria were evaluated. Experimental data on melt supercooling were used to construct a metastable liquidus above the peritectic temperatures. It is shown that the peritectically forming compounds LiB3O5 and Li4B10O17 can, under certain conditions, melt congruently.