The BaO · 2B
2O
3–La
2O
3 · 3B
2O
3 join of the ternary system BaO–La
2O
3–B
2O
3 is studied using differential thermal analysis, x-ray diffraction, and density measurements. The join is shown to be pseudobinary, with eutectic phase relations. In the composition range 5–99.8 mol % La
2O
3 · 3B
2O
3
, glassy materials are obtained, which seems to be associated with the existence of the congruently melting compound BaO · La
2O
3 · 5B
2O
3.