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The fifteen intermetallic compounds R4Pd9Al24 (R = Gd–Tm) and R4Pt9Al24 (R = Y, Gd–Lu) were prepared by reaction of the elemental components. Their crystal structure was determined from single‐crystal X‐ray data of Er4Pt9Al24. It is pseudo‐trigonal with triclinic symmetry: P 1, a = b = 747.5(2) pm, c = 1306.7(4) pm, α = 100.99(2)°, β = 95.47(2)°, γ = 60.00(3)°, Z = 1, R = 0.052 for 2593 structure factors and 110 variable parameters. The structure is closely related to that of Y2Co3Ga9. Both may be described as stacking variants of each other. They consist of layers of the compositions PtAl2 (CoGa2), and Er2Al3 (Y2Ga3), designated A and B, respectively. These layers are stacked in the five‐ and four‐layer sequences ABAAB (Er4Pt9Al24) and ABAB (Y2Co3Ga9). The layers PtAl2 and CoGa2 are similar to the hexagonal close packed layers in the TiSi2‐, CrSi2‐, and MoSi2‐type structures. The structure of Er4Pt9Al24 contains a monoclinic subcell, where the layers Er2Al3 are disordered. A partial disorder of this kind, which could be ascribed to twinning or to the intergrowth with another stacking variant, was found during the structure refinement of the isotypic compound Y4Pt9Al24: a = b = 749.0(2) pm. c = 1309.3(4) pm, α = 100.99(2)°, β = 95.48(2)°, γ = 60.00(3)°, R = 0.031 for 1435 structure factors and 128 variable parameters.

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