AbstractMixed Alkaline Earth Aluminides “AIIAl2”Mixed alkaline earth aluminides near the composition A1IIxA2II1–xAl2 (AII = Ca, Sr, Ba) were synthesized from stoichiometric ratios of the elements in tantalum crucibles; their crystal structures were determined using single‐crystal X‐ray data. Starting from the binary cubic Laves phase CaAl2 (|:ABC:| stacking of aluminium Kagomé nets) up to 73 % of calcium can be replaced by strontium (Ca0.27Sr0.73Al2, cubic, space group , a = 821.2(2) pm; Z = 8, R1 = 0.0083). The new ternary compound Ca1.1Sr5.9Al13 (trigonal, space group , a = 582.35(3), c = 1655.2(2) pm, Z = 1, R1 = 0.0387) forms the structure type originally reported for ‘Ba7Al13’, which exhibits blocks of three Kagomé nets ABC as a section of the MgCu2 structure. Blocks of two connected Kagomé nets (Sr5Al9 structure type: trigonal, space group , Z = 3) are stable from a Sr proportion of approx. 90 % (Ca0.5Sr4.5Al9: a = 582.61(5), c = 3549.0(6) pm, R1 = 0.0512) through the binary Sr phase up to the barium substituted compound Sr3.9Ba1.1Al9 (a = 590.88(2), c = 3585.2(2) pm, R1 = 0.0212). This structure type is also observed on the Ca–Ba section i.e. at a comparable merged cation radius, between Ca3.3Ba1.7Al9 (a = 578.91(14), c = 3537.7(13) pm, R1 = 0.0462) and Ca2.6Ba2.4Al9 (a = 582.23(6), c = 3570.2(7) pm, R1 = 0.0309). Starting from the binary Ba compound Ba21Al40 (trigonal, space group P31m, Z = 1), only a small Ca (Ca1.9Ba19.1Al40: a = 1048.5(2), c = 1718.3(4) pm, R1 = 0.0624) and Sr substitution (Sr4.3Ba16.7Al40 (a = 1048.64(11), c = 1713.1(2) pm, R1 = 0.0242) is possible without a structure change. Also only a small amount of aluminium can be substituted by Ge (Ba21Al39.2Ge0.8: a = 1053.10(13), c = 1718.4(2) pm, R1 = 0.0576) or In (Ba21Al35.3In4.7: a = 1061.10(9), c = 1738.1(2) pm, R1 = 0.0569) in the latter structure. In this series of compounds near the composition AAl2, the radii of the cations appears to be the decisive element, that governs structure and to some extent even the composition. The available tolerances and the individual site distributions are discussed. For an electronic comparison, FP‐LAPW band structure calculations are performed for binary aluminides of all observed structure types and the results are compared to those of the simple electron counting methods.