The structures of the new heavy fermion materials CeIrIn 5 and CeRhIn 5 have been determined by single-crystal neutron (R(F) β«Ψβ¬ 0.051) and X-ray (R(F) β«Ψβ¬ 0.056) di4raction, respectively. Both compounds adopt tetragonal structure, space group P4/mmm (No. 123), Z β«Ψβ¬ 1, a β«Ψβ¬ b β«Ψβ¬ 4.674(1) A > , and cβ«)5(105.7Ψβ¬ A > for CeIrIn 5 , and a β«Ψβ¬ b β«Ψβ¬ 4.656(2) A > , and c β«Ψβ¬ 7.542(1) A > for CeR-hIn 5 . The possible presence of antisite disorder, a long-standing question on this type of structure, was excluded by both electron and neutron or X-ray di4raction. The compounds are built by monolayers of face-sharing distorted cuboctahedra [CeIn 3 ] and monolayers of edge-sharing rectangular parallelepipeds [RhIn 2 ] or [IrIn 2 ], stacked alternatively in the [001] direction. Therefore, they are new members of the inhomogeneous linear homologous series M m T n T 3mΨ2n . Because of their ordered intergrowth structure, the physical properties of the quasi-two-dimensional heavy electron systems CeIrIn 5 and CeRhIn 5 can be directly compared with the corresponding ones of their parent compound, the three-dimensional heavy fermion material CeIn . 2001