A new type of lithium rare earth oxyborate of formula Li 2 Ln 5 O 4 (BO 3 ) 3 (Ln ؍ Yb, Lu) has been discovered in the ternary phase diagrams Li 2 O+Ln 2 O 3 +B 2 O 3 . Single crystals of Li 2 Yb 5 O 4 (BO 3 ) 3 were grown by the 6ux method. The structure of this oxyborate was solved from a data collection with a four circle automatic di4ractometer and with MoK radiation. The cell is monoclinic (space group P2 1 /m, Z ؍ 2) with a ؍ 10.095(2) A > , b ؍ 3.519(2) A > , c ؍ 15.647(11) A > , ؍ 105.45(3)3, V ؍ 535.7(5) A > ؊3 . Re5nement of 86 parameters using 3298 independent re6ections having intensity I> 2 (I) led to R ؍ 0.037 (wR ؍ 0.091). The structure of Li 2 Yb 5 O 4 (BO 3 ) 3 is made up of a bidimensional framework of edge or corner-sharing YbO 7 polyhedra parallel to the (101 ) plane and connected by threecoordinated boron atoms and lithium ions. It can also be described as a bidimensional assembly of OYb 4 tetrahedra, ((Yb 10 O 8 ) 14؉ ) n , separated by borate groups and lithium coordination polyhedra forming ribbons, ((Li 4 B 6 O 18 ) 14؊ ) n , along the b direction. The presence of oxygen atoms sharing only Yb polyhedra justi5es the oxyborate label.