Quaternary ammonium salts with the carborane anion CB11H12- ([cation][CB11H12]; cations: Me4N+, Et4N+, (C4H8)(2)N+, Bu4N+, and MeEt3(MeOCH2CH2)N+) were prepared. Each salt displayed several solid phase transitions and exhibited an ionic plastic phase at high temperatures. Crystallographic analysis revealed that the disorder of the cation conformation and anion orientation become more extensive in the higher temperature phases.