Mixed crystal Laves phases Ln1–xEuxMg2 (Ln = La, Sm) were prepared from the elements by melting reactions and characterized by X-ray powder diffraction. In the substitutional range 0 ≤ x ≤ 0.5 cubic Laves phases of the MgCu2 type, for 0.8 ≤ x ≤ 1 (Ln = La) and 0.9 ≤ x ≤ 1 (Ln = Sm), respectively, hexagonal Laves phases of the MgZn2 type are formed, separated from each other by a miscibility gap. The dependence of the lattice parameters on the degree of substitution x follows closely a Vegard type behavior with a(La1–xEuxMg2 (0 ≤ x ≤ 0.5)) = (8.798(3) + 0.00177(8)x) Å and a(Sm1–xEuxMg2 (0 ≤ x ≤ 0.5)) = (8.629(2) + 0.00404(7)x) Å. Analysis of the unit cell volumes suggests europium to be divalent in all mixed Laves phases. Hydrogenation of the Laves phases as followed by in situ thermal analysis is exothermic for all compositions. For the hexagonal europium rich Laves phases the reaction products upon hydrogenation at 6 MPa hydrogen pressure in an autoclave are binary lanthanum hydride and EuMg2H6, the latter being unsubstituted according to lattice parameters refined by Rietveld analysis. For lanthanum-rich compounds (0 ≤ x ≤ 0.1) ternary hydrides of the tetragonal LaMg2D7 type structure are formed, if the hydrogen pressure is high enough (11 MPa), and heating rates and reaction temperatures are kept low (373 K). The crystal structure of the new hydride La0.9Eu0.1Mg2Hy≤7 was refined from X-ray powder diffraction data by Rietveld analysis (space group P41212, a = 6.2592(1) Å, c = 9.3759(3) Å, V = 367.322(2) Å3, Z = 4).