The cesium-richest phase in the Cs-Tl system, CsTl, can be isolated as a pure crystalline phase through slow cooling of cesium-richer compositions in Ta followed by vacuum sublimation of the excess Cs at similar to 100 degrees C. The compound melts incongruently in the neighborhood of 150 degrees C. The structure was established by single crystal X-ray diffraction at room temperature (orthorhombic Fddd, Z = 48, a = 32.140(3) Angstrom, b = 15.136(1) Angstrom, and c = 9.2400(7) Angstrom. The isolated Tl-6(6-) ions in the structure, tetragonally compressed octahedra, exhibit D-2 symmetry with less than or equal to 0.063(4) Angstrom differences in edge lengths from D-4h. Extended Huckel calculations confirm the classical nature of the distortion in lowering the cluster electron count and charge. The diamagnetic CsTl differs from KTl, which contains more distorted Tl-6(6-) ions (C-2h), in a doubled a axis and an additional interpenetrating cluster anion sublattice. The smaller distortion of the clusters in CsTl originates with the lower field of the same number of cation neighbors in a more symmetric arrangement. The structural trend from LiTl (CsCl type) through the classic NaTl (stuffed diamond) to the related KTl and CsTl with compressed octahedra (and the absence of any such Rb phase) relates to the efficient packing of A(+) and (Tl-)(n) over a widely varying range of A(+) sizes, with only Na+ fitting well within a (3)(infinity)(Tl-) lattice. Similarly 4-bonded thallium in KTl and CsTl with somewhat shorter Tl-Tl distances is a necessary alternative when the infinite lattice cannot tolerate larger cations.