Abstract The methanolothermal reaction of Cs 2CO 3with Se in the presence of Bi 2Te 3at 160°C yields two tellurium-poor mixed polychalcogenides of the general type Cs 4Te x Se 16− x . Cs 4TeSe 15is isostructural with Cs 4Se 16and crystallizes in the orthorhombic space group Pbcmwith a=10.042(2), b=14.029(3), c=19.120(4) Å, Z=4, and V=2693.6(10) Å 3. It contains discrete [TeSe 15] 4−anions, in which a central Te atom is coordinated in a distorted square-planar manner by three Se 2− 5chains, one of which is chelating. Cs 2Te 2Se 6crystallizes in the monoclinic space group P2 1/ cwith a=7.436(1), b=6.771(1), c=13.489(3) Å, β=95.55(3)°, Z=2, and V=676.0(2) Å 3and is isostructural with Cs 2Te 8. If distances longer than 2.90 Å are ignored, the anionic structure may be described as a discrete [Te 2Se 6] 2−chain in which the Te atoms adopt the third and sixth positions. Individual chains are connected through Te···Se interactions of lengths 2.922(2) and 3.244(1) Å into sheets. When Sb 2Te 3is employed in place of Bi 2Te 3, reaction of Cs 2CO 3with Se yields Cs 4[TeSe 3] 2·Te 3Se 3·1.5(Te 3.21Se 2.79) in which pyramidal [TeSe 3] 2−anions and respectively ordered and disordered Te x Se 6− x rings are connected through secondary Te···Se bonds into polymeric layers. Cs 4Te 9.74Se 13.26crystallizes in the rhombohedral space group R 3 with a= b=12.022(2), c=44.072(9) Å, Z=6, and V=5516(2) Å 3.