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Abstract: Quaternary chalcogenides InSn2Bi3Se8 and In0.2Sn6Bi1.8Se9 were synthesized on direct combination of their elements in stoichiometric ratios at T>800°C under vacuum. Their structures were determined with X-ray diffraction of single crystals. InSn2Bi3Se8 crystallizes in monoclinic space group C2/m (No. 12) with a=13.557(3)Å, b=4.1299(8)Å, c=15.252(3)Å, β=115.73(3)°, V=769.3(3)Å3, Z=2, and R1 /wR2 /GOF=0.0206/0.0497/1.092; In0.2Sn6Bi1.8Se9 crystallizes in orthorhombic space group Cmc21 (No. 36) with a=4.1810(8)Å, b=13.799(3)Å, c=31.953(6)Å, V=1843.4(6)Å3, Z=4, and R1/wR2/GOF=0.0966/0.2327/1.12. InSn2Bi3Se8 and In0.2Sn6Bi1.8Se9 are isostructural with CuBi5S8 and Bi2Pb6S9 phases, respectively. The structures of InSn2Bi3Se8 and In0.2Sn6Bi1.8Se9 feature a three-dimensional framework containing slabs of NaCl-(311) type with varied thicknesses. Calculations of the electronic structure and measurements of electrical conductivity indicate that these materials are semiconductors with narrow band gaps. Both compounds show n-type semiconducting properties with Seebeck coefficients −270 and −230μV/K at 300K for InSn2Bi3Se8 and In0.2Sn6Bi1.8Se9, respectively. [Copyright &y& Elsevier]