The new compounds K(2)Ln(2)As(2)Se(9) (Ln = Sm, Gd) were obtained by applying the reactive flux method, The structure consists of a three-dimensional (M) [Ln(2)As(2)Se(9)](2-) framework with K+ ion-filling tunnels running along the b axis. The two unique Ln(3+) cations are coordinated by two Se-2(2-) dumbbells, two AsSe33-pyramids, and one chairlike As2Se42- unit in a bicapped trigonal-prismatic geometry. The Ln(3+)-centered trigonal prisms share triangular faces with neighboring prisms, forming one-dimensional chains along the b axis. These chains are linked to each other to form layers by sharing Se2- anions on the capped sites of the trigonal prisms. The As2Se4 units connect these layers to form the 3D framework.