Various samples of the solid solution Gd₄NiMg_1−<i>x</i>Al<i>_x</i> were synthesized from the elements in sealed tantalum ampoules in an induction furnace. These gadolinium-rich compounds crystallize with the cubic Gd₄RhIn type structure, space group the F4-; 3m. All samples were characterized on the basis of Guinier powder patterns. The lattice parameter decreases with increasing aluminium content in a <i>Vegard</i>-like manner. The structures of two crystals were refined on the basis of diffractometer data: <i>a</i> = 1359.3(1), w<i>R</i>2 = 0.0711, 481 <i>F</i>² values, 20 variables for Gd₄NiMg_0.52(1)Al_0.48(1) and <i>a</i> = 1351.40(6), w<i>R</i>2 = 0.0665, 478 <i>F</i>² values, 21 variables for Gd₄NiMg_0.10(1)Al_0.90(1). The main structural motif is a rigid three-dimensional network of edge- and corner-sharing Gd₆Ni trigonal prisms and isolated (Mg/Al)₄ tetrahedra in fcc packing. The edge length of the tetrahedra drastically decreases from Gd₄NiMg (309 pm) to Gd₄NiMg_0.10(1)Al_0.90(1) (294 pm). The Gd₄NiMg_1−<i>x</i>Al<i>_x</i> samples show Curie–Weiss behaviour with slightly higher magnetic moment values than the theoretical one for a free Gd³⁺ ion. By substitution of Gd₄NiMg with aluminium the cell volume decreases and the atomic disorder (Mg/Al) induces a random distribution of the magnetic interactions. This leads to a shift of the Néel temperatures to lower values when increasing the aluminium content. Already for Gd₄NiMg_0.4Al_0.6 the long-range antiferromagnetic ordering no longer exists and the system comes into a cluster-glass state.< Réduire