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The optical reflectance of BiSI has been measured with photon energy, from 2.2 to 3.6 eV. The band structures of BiSBr, BiSCl, BiSeBr, BiSeCl and BiSeI have been calculated by using the self-consistent pseudopotential method. These results show that: (i) they are indirect gap semiconductors; (ii) the top of the valence band is located at some k-point along rZ (the z-direction) with finite curvature (small effective mass); (iii) the highest valence band at r (the center of the Brillouin zone) is only a few tenths of an eV lower than the energy of the top of the valence band and is flat (large effective mass) especially in the directions perpendicular to z. (iv) in addition to the above listed features, the minimum of the conduction band for BiSCl and BiSeCl is located at some k-point along rX, the x direction and is a few tenths of an eV lower than the lowest conduction band at r. These results suggest that: All the six compounds studied in this project will exhibit the Gunn effect if they are properly doped. They are potential candidates for microwave oscillators. (Author)