New intermetallic compounds Ir 3Mg 13− x In x ( x=2.37 and 3.36) were synthesized by induction melting of the elements in glassy carbon or tantalum crucibles in a water-cooled sample chamber. Both compounds were investigated by X-ray powder diffraction and the structures have been refined on the basis of single-crystal diffractometer data: R 3 ̄ c , a=1606.2(1), c=843.37(7) pm , w R 2=0.0689, 928 F 2 values, 33 variables for x=2.37 and a=1608.1(1), c=843.32(6) pm, w R 2=0.0366, 929 F 2 values, 33 variables for x=3.36. The complex crystal structure has only one iridium site with coordination number CN=11. The four crystallographically independent magnesium sites show all mixed Mg/In occupancy with between 3% and 41% indium. The structure can be described by an intergrowth of CN11, 14, 15, 14, and 12 polyhedra of Ir, Mg1/In1, Mg2/In2, Mg3/In3, and Mg4/In4, respectively.