Two new alkali metal thiotrielates with the general composition M4T2S5 (M = Rb, Cs; T = Al, In) were obtained via the azide route and their crystal structures were determined from single-crystal X-ray diffraction data. Highly air sensitive Rb4Al2S5 crystallizes in pale red rods in the space group Pna2(1) (no. 33) with lattice constants a = 11.4450(5) angstrom, b = 11.4605(6) angstrom, c = 9.5727(4) angstrom, and a cell volume of V = 1255.6(1) angstrom(3) with Z = 4. To the best of our knowledge, Rb4Al2S5 represents the first rubidium thioaluminate and a new structure type in the field of ternary alkali metal chalkogenometallates. Cs4In2S5 crystallizes isotypic to Rb4In2S5 in the space group P (1) over bar (no. 2) with lattice constants a = 9.6871(3) angstrom, b = 11.1549(4) angstrom, c = 7.7879(3) angstrom, = 110.731(3)degrees, = 95.760(3)degrees, = 102.766(2)degrees, and a cell volume of V = 752.75(4) angstrom(3) with Z = 2. The crystal structures of both solids consist of edge-linked [T2S6](6-) double tetrahedra, which build up one-dimensional, polymeric (1)[T2S54-] strands by trans-corner sharing. The anionic chains are separated by rubidium and cesium cations, respectively. Aluminum as well as indium has a slightly distorted tetrahedral surrounding by sulfur, and the four independent rubidium atoms in Rb4Al2S5 feature distorted trigonal prismatic and distorted octahedral coordination spheres, respectively. In Cs4In2S5 the four different cesium positions show coordination numbers of six and seven. Cs4In2S5 was also characterized by Raman spectroscopy. The frequencies of In-S stretching modes are found in the range from 342 cm(-1) to 292 cm(-1).