The rare earth metal rich compounds RE 4 NiMg (RE ¼ Y, Pr-Nd, Sm, Gd-Tm, Lu) were synthesized from the elements in sealed tantalum tubes in an induction furnace. All compounds were investigated by X-ray diffraction on powders and single crystals: Gd 4 RhIn type, space group F4 ¯3m, Z ¼ 16, a ¼ 1367.6(2) pm for Y 4 NiMg, a ¼ 1403.7(3) pm for Pr 4 NiMg, a ¼ 1400.7(1) pm for Nd 4 NiMg, a ¼ 1386.5(2) pm for Sm 4 NiMg, a ¼ 1376.1(2) pm for Gd 4 NiMg, a ¼ 1362.1(1) pm for Tb 4 NiMg, a ¼ 1355.1(2) pm for Dy 4 NiMg, a ¼ 1355.2(1) pm for Ho 4 NiMg, a ¼ 1354.3(2) pm for Er 4 NiMg, a ¼ 1342.9(3) pm for Tm 4 NiMg, and a ¼ 1336.7(3) pm for Lu 4 NiMg. The nickel atoms have trigonal prismatic rare earth coordination. These NiRE 6 prisms are condensed via common edges to a three-dimensional network which leaves voids for Mg 4 tetrahedra and the RE1 atoms which show only weak coordination to the nickel atoms. The single crystal data indicate two kinds of solid solutions. The RE1 positions reveal small RE1/Mg mixing and some compounds also show Ni/Mg mixing within the Mg 4 tetrahedra. Y 4 NiMg and Gd 4 NiMg have been tested for hydrogenation. These compounds absorb up to eleven hydrogen atoms per formula unit under a hydrogen pressure of 1 MPa at room temperature. The structure of the metal atoms is maintained with only an increase of the lattice parameters (DV/VE22%) if the absorption is done at To363 K as at higher temperature a decomposition into REH 2 -REH 3 hydrides occurred. Moreover, the hydrogenation affects drastically the magnetic properties of these intermetallics. For instance, Gd 4 NiMg exhibits an antiferromagnetic behavior below T N ¼ 92 K whereas its hydride Gd 4 NiMgH 11 is paramagnetic down to 1.8 K.