A series of binary rare-earth metal silicides RE 5 Si 3 and ternary boron-interstitial phases RE 5 Si 3 B x (RE ¼ Gd, Dy, Ho, Lu, and Y) adopting the Mn 5 Si 3 -type structure, have been prepared from the elemental components by arc melting. Boron ''stuffed'' phases were subsequently heated at 1750 K within a high-frequency furnace. Crystal structures were determined for both binary and ternary series of compounds from single-crystal X-ray data: hexagonal symmetry, space group P6 3 /mcm, Z ¼ 2. Boron insertion in the host binary silicides results in a very small decrease of the unit cell parameters with respect to those of the binaries. According to X-ray data, partial or nearly full boron occupancy of the interstitial octahedral sites in the range 0.6-1 is found. The magnetic properties of these compounds were characterized by the onset of magnetic ordering below 100 K. Boron insertion induces a modification of the transition temperature and y p values in most of the antiferromagnetic binary silicides, with the exception of the ternary phase Er 5 Si 3 B x which was found to undergo a ferromagnetic transition at 14 K. The electrical resistivities for all binary silicides and ternary boron-interstitial phases resemble the temperature dependence of metals, with characteristic changes of slope in the resistivity curves due to the reduced electron scattering in the magnetically ordered states. Zintl-Klemm concept would predict a limiting composition RE 5 Si 3 B 0.6 for a valence compound and should then preclude the stoichiometric formula RE 5 Si 3 B. Density functional theory calculations carried out on some RE 5 Si 3 Z x systems for different interstitial heteroatoms Z and different x contents from 0 to 1 give some support to this statement.