The system Ba-Tl-Cl-O has been investigated in order to get a better understanding of the rich chemistry of Tl(III). The new oxychloride Ba3Tl2O5Cl2 has been synthesized, the structure of which has been determined by a combined HREM and powder X-ray diffraction study. This phase, which crystallizes in the space group I4/mmm (a = 4.3499(2) Angstrom, c = 25.367(1) Angstrom), can be described either as a strongly distorted Sr3Ti2O7 structure or as an intergrowth of rock salt layers [Ba2Cl2](infinity) with [BaTl2O5](infinity) layers similar to the [BaFeCuO5](infinity) layers observed in the YBaFeCuO5 structure. The regular TlO5 pyramids are emphasized with respect to the strongly distorted TlO5Cl octahedra.