A new anionic thallium cluster chain (1)(infinity)[Cd2TI115-] has been discovered in the A-Cd-Tl systems for A = Cs, Rb. The compounds are synthesized by direct fusion of the elements at 700 degrees C and equilibration of the quenched product at 200 degrees C for 1 month. The thallides crystallize in the orthorhombic space group Amm2, Z = 2, a = 5.6107(7) and 5.5999(6) Angstrom, b = 18.090(3) and 17.603(3) Angstrom, c = 13.203(3) and 12.896(2) A for A = Cs and Rb, respectively, and contain chains of fate-sharing pentagonal Tl-10 antiprisms embedded in a matrix of alkali metal cations. Cadmium atoms occupy the center of the antiprisms and donate electrons to the anionic chain. Additional four-bonded Tl atoms on one side of the chain make the structure acentric. The compounds are diamagnetic (chi(296) = -0.8, -4.0 (x 10(-4)) emu/mol, respectively) and metallic (10-20 mu Omega cm at 275 K), and the indirect band gap energy of both compounds is close to zero according to extended Huckel calculations on the isolated chain.