A new modification of the cyclopentadienylbismuth dihalide [(C5HR4)BiCl(m-Cl)]2 (1) (R = CHMe2) has been characterized by x-ray diffraction. The coordination of the five-membered ring as well as the arrangement of the chloro bridges differ from those of the known modification. On reaction of 1 or its pentaalkylated deriv. [(C5Me2R3-1,2,4)BiCl(m-Cl)]2 (2) with AlCl3, the bismuthenium salts [(C5Me2R3)Bi(m-Cl)]22- [AlCl4-]2 (3) and [(C5HR4l)Bi(m-Cl)]22+ [AlCl4-]2 (4) have been obtained. Both are centrosym. dimers without solvent coordination. Only for 4 a very weak interaction with the counterion could be defected. The crystal structure of 1, 3, and 4 were detd.