In the system TlI-AsI3 we find no ternary halide. The phase diagram TlI-BiI3 shows the two stable compounds Tl3Bi2I9 and Tl7Bi3I. Twinning prevents a full determination of the structures. Large single crystals of Tl7Bi3I are obtained by the Bridgman technique. By sensitive piezoelectric and electrooptic measurements the absence of a center of symmetry is demonstrated; however, the effects are strongly quenched by the structural twinning. The electronic properties are similar to those of BiI3. A direct band gap at about 2.2 eV is accompanied by two indirect transitions at slightly smaller photon energies.