To obtain rare earth luminescent materials with weak concentration quenching, the B 2O 3-rich portion of the ternary diagram Ln 2O 3MgOB 2O 3 ( Ln = rare earth) has been investigated. A ternary phase of composition LnMgB 5O 10 has been found for Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er. These compounds all crystallize in the monoclinic space group P2 1 c . The structure has been determined on a LaMgB 5O 10 crystal. A full-matrix least-squares refinement leads to R = 0.039. The structure can be described as being made of (B 5O 10 5−) n two-dimensional layers linked together by the lanthanum and magnesium ions. The rare earth atom coordination polyhedra form isolated chains. These borates are isostructural with some rare earth cobalt borates.