Solid state phase equilibria in the ternary Gd–Si–B phase diagram have been proposed at 1270 K using X-ray diffraction, scanning electron microscopy and electron probe microanalysis. Prior to this work, the binary systems Gd–B, Gd–Si and Si–B have also been reinvestigated. The main characteristic of the ternary diagram is the occurrence of two new ternary compounds Gd 5Si 2B 8 and Gd 5Si 3B 0.64. The former crystallizes in tetragonal symmetry, space group P4/ mbm with unit cell parameters a=7.2665(3), c=8.2229(7) Å, the second one presents hexagonal symmetry, space group P6 3/ mcm with unit cell parameters a=8.5080(4), c=6.4141(2) Å. The X-ray structures of the two structurally related phases Gd 5Si 3B 0.64 and host binary Gd 5Si 3 have been refined from three-dimensional single-crystal intensity data to the final R values of 0.036 ( R w=0.046) and 0.046 ( R w=0.055) for 457 and 401 reflections, respectively with [ F>4 σ( F)]. Both structures exhibit the Mn 5Si 3-type structure, with in addition for Gd 5Si 3B 0.64 a partial occupancy by boron of the normally vacant interstitial site at the center of the Gd 6 octahedron, which corresponds to the origin of the unit cell. Bonding between the interstitial boron atoms and the gadolinium ones forming the Gd 6B polyhedra is indicated by the decrease in the corresponding Gd–Gd distances and consequently in the unit cell volume. Finally, the Gd–Si–B phase diagram is compared with the previously reported Er–Si–B, at 1070 K.