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The introduction of Bi as a substitution for In at ~12% in In 2 Se 3 stabilizes the α-polymorph and facilitates the crystal growth by the modified Bridgeman method. The crystal structure ( R −3 m , Z = 3, a = 3.9978(8) A, c = 28.276(6) A) and composition, (In 0.88 Bi 0.12 ) 2 Se 3 , of the crystals were determined by single crystal X-ray diffraction. The structure, of the tetradymite type, displays positional disorder within the middle Se layer. Optical measurements indicate that (In 0.88 Bi 0.12 ) 2 Se 3 has an indirect band gap of about 1.19 eV, shown by electronic structure calculations to be from valence band states near the Γ point to conduction band states at the L point. Resistivity and Hall effect measurements on Sn-doped crystals of composition (In 0.88 Bi 0.115 Sn 0.005 ) 2 Se 3 show it to have a relatively high semiconducting resistivity, about 6 × 10 4 Ω cm at 300 K, with an n -type carrier concentration varying from 10 12 /cm 3 at 300 K to 10 15 /cm 3 at 400 K.