In the title compound, CH3(C6H5)(3)P+. C3H13B10S-, the negative charge of the sulfide group connected to the carborane cage is balanced by the phosphonium cation. In the cation, one of the three phenyl groups is disordered, exhibiting two orientations. In the anion, the C-cage-C-cage distance is 1.792 (5) Angstrom.