The title compound is synthesized by direct fusion of the elements at 400 degrees C followed by annealing at 330 degrees C, quenching to room temperature, and subsequent annealing at 120 and 100 degrees C for days to weeks. The compound crystallizes in the monoclinic space group P2(1)/c (No. 14), with Z = 4, a = 10.132(1) Angstrom, b = 22.323(2) Angstrom, c = 13.376(1) Angstrom, and beta = 93.14(1)degrees, and consists of Tl-7(7-) clusters embedded in a matrix of potassium ions. The cluster is an axially compressed pentagonal bipyramid close to D-5h symmetry. The apex-apex bond distance (3.462(1) Angstrom) is little longer than the bonds in the pentagonal waist (3.183(1)-3.247(1) Angstrom). Structurally the compound is not electron-precise: K10Tl7 has three extra electrons per Tl-7 cluster and is Pauli paramagnetic (chi(300) = 2.25 x 10(-4) emu/mol).