Quaternary chalcogenides Pb x Sn 6−x Bi 2 Se 9 (x=0–4.36) were synthesized with solid-state methods; their structures were determined from the X-ray diffraction of single crystals. Pb x Sn 6−x Bi 2 Se 9 crystallizes in an orthorhombic space group Cmcm (No. 63); the structure features a three-dimensional framework containing slabs of NaCl-(311) type that exhibits identical layers containing seven octahedra units, which expand along the direction [010]. Each slab contains fused rectangular units that are connected to each other with M–Se contacts in a distorted octahedral environment. Calculations of the band structure, measurements of Seebeck coefficient and electrical conductivity confirm that these compounds are n-type semiconductors with small band gaps and large electrical conductivities.