Quaternary chalcogenides PbxSn6-xBi 2Se9 (x=04.36) were synthesized with solid-state methods; their structures were determined from the X-ray diffraction of single crystals. PbxSn6-xBi2Se9 crystallizes in an orthorhombic space group Cmcm (No. 63); the structure features a three-dimensional framework containing slabs of NaCl-(3 1 1) type that exhibits identical layers containing seven octahedra units, which expand along the direction [0 1 0]. Each slab contains fused rectangular units that are connected to each other with MSe 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.