The new isostructural compounds, K 2 Gd 2 Sb 2 Se 9 and K 2 La 2 Sb 2 S 9 , were discovered by the molten polychalcogenide salt method. They crystallize in the orthorhombic space group Pbam with a β«Ψβ¬ 11.4880(3) A s , b β«Ψβ¬ 17.6612(1) A s , c β«Ψβ¬ 4.2201(1) A s , and Z β«Ψβ¬ 2 for K 2 Gd 2 Sb 2 Se 9 and a β«Ψβ¬ 11.2080(5) A s , b β«Ψβ¬ 16.8781(8) A s , c β«)2(9142.4Ψβ¬ A s , and Z β«Ψβ¬ 2 for K 2 La 2 Sb 2 S 9 . The compounds have a three-dimensional [M 2 Sb 2 Q 9 ] 2Ψ framework (M β«Ψβ¬ Gd, La; Q β«Ψβ¬ Se, S) with K Ψ -ion-5lled channels running along the c axis. The coordination geometry around the rare earth atom is best described as a bicapped trigonal prism. The Gd 3Ψ /La 3Ψ centered trigonal prisms share triangular faces with neighboring prisms forming one-dimensional columns along the c axis. The columns are connected to each other to form sheets by sharing Se/S atoms on the capping sites of the trigonal prisms. Sb 3Ψ ions are stabilized in distorted octahedral sites. The SbQ 6 (Q β«Ψβ¬ Se, S) octahedra share edges with neighboring octahedra making an in5nite chain along the c axis and bridging Gd/La layers together to make the whole framework three-dimensional. In both compounds, Sb atoms appear to be positionally disordered over two crystallographically different sites with half occupancy. This disorder was removed upon elucidation of a 2aΨ2bΨ2c superstructure, which more accurately describes the positional ordering of Sb atoms in the structure. The superstructure of K 2 Gd 2 Sb 2 Se 9 was re5ned in the monoclinic space group C2/m with a β«Ψβ¬ 22.8783(4) A s , b β«Ψβ¬ 8.4062(2) A s , c β«Ψβ¬ 20.970(1) A s , β«Ψβ¬ 123.022(1)3, and Z β«Ψβ¬ 8. These compounds are semiconductors with band gap values of 1.