HighTcsuperconducting materials with nominal compositions of Tl1‐xBix(Sr, Ba)2Ca2Cu3O9(0≤x≤0.5) have been investigated using x‐ray powder diffractometry and electron microscopy. It was found that Tl and Bi cations were in an ordered arrangement in the (Tl, Bi)–O atomic layer. Although the proposed supercell is 4a×4b×4cderived from the basic unit cell of the undoped TlBa2Ca2Cu3O9(Tl‐1223), the basic two‐dimensional superlattice in the (Tl, Bi)–O layer is 2a×4bor 4a×2bwith Bi cations at the center and the corner of the rectangle, giving an ideal and preferred composition with the Tl:Bi ratio of 3:1. Accordingly, the maximum Bi doping for Tl in the solid solution is 25%. The proposed model of the superstructure was supported by computer simulation of electron diffraction patterns. The mechanism of formation of the superstructure is discussed. ©1996 American Institute of Physics.