Reaction of the elements in tantalum containers at 450 degrees C followed by slow cooling yields the title phase, which crystallizes in the hexagonal space group P6(3)/mmc (No. 194), Z = 2, n = 11.136(6) Angstrom, c = 29.352(7) Angstrom. The compound contains separate layers of Tl8Cd310- and Tl-5(7-) clusters separated by Na+ and KC cations, viz., (Na+)(9)(K+)(16)(Tl-5(7-))(2)(Tl8Cd310-)e(-) The new Tl8Cd310-is a pentacapped trigonal prism (D-3h), isosteric and isoelectronic with Tl-11(7-), With Cd substituted in the waist-capping positions, the vertexes with the least negative Mulliken charge in the parent cluster. The compound is semiconducting and Curie-Weiss paramagnetic (-1.50 mu(B)), these evidently reflecting the extra electron trapped in some alkali-metal-bordered cavity. Some energetics of the atom substitution reaction are considered.