The La–Ni–Mg isothermal section at 500°C was investigated in the region from 66.7 to 100at.% Ni by means of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDXS) and X-ray powder diffraction (XRPD). Phase relations were established in this composition range.The binary boundary phases MgNi 2 , LaNi 5 and La 7 Ni 16 do not dissolve any appreciable amount of the third element. Three solid solutions were found to exist, extending at a constant Ni content. The range of homogeneity (0<x≤0.60) and crystal structure (cF24-MgCu 4 Sn) of (La 1−x Mg x )Ni 2 were confirmed. (La 1−x Mg x ) 2 Ni 7 (0≤x≤0.13) derives from the binary La 2 Ni 7 (L.T.), of hexagonal hP36-Ce 2 Ni 7 type structure. (La 1−x Mg x )Ni 3 (0≤x≤0.67) forms from the binary LaNi 3 , whose structure is trigonal hR36-PuNi 3 . For x=0.67 the stoichiometry LaMg 2 Ni 9 is obtained, which was already reported in the literature as a ternary ordered phase with PuNi 3 type structure.