New radical cation salts (BEDT-TTF)[8,8',(7)-Cl₂(Cl_0.09)-3,3'-Co(1,2-C₂B₉H_9.91)(1',2'-C₂B₉H₁₀)] (1), (BEDT-TTF)[8,8'-Br_0.75Cl_1.25-3,3'-Co(1,2-C₂B₉H₁₀)₂] (2), and (BMDT-TTF)₄[8,8'-Br_1.16(OH)_0.72-3,3'-Co(1,2-C₂B₉H_10.06)₂] (3) were synthesized, and their crystal structures and electrical conductivities were determined. All the radical cation salts are semiconductors. Compounds 1 and 2 were found to be isostructural, however their electrical conductivities strongly differ (s₂₉₃ = 2 Ω⁻¹cm⁻¹ and 10⁻⁵ Ω⁻¹cm⁻¹, respectively).