Dimethyltrithiocarbonate reacts with the superacidic systems HF/MF5 and DF/MF5 (M = As, Sb) to form (MeS)(2)CSX+MF6- (X = H, D). With an excess of AsF5 the reaction leads to (MeS)(2)CSX+As2F11- (X = H, D). The latter salt crystallizes in the monoclinic space group P2(l)/c (No. 14 with a = 7.687(1) Angstrom, b 221.327(6) Angstrom, c = 8.704(1) Angstrom, and beta = 91.99(1)degrees) with four formula units per cell. The undecafluorodiarsenate anion contains an As-F-As bridge with two As-F bonds significantly longer than the nonbridging bonds and an As-F-As angle of 159.1(6)degrees. The Raman, IR, and H-1, As-75, F-19, and C-13 NMR spectra of (MeS)(2)CSX+SbF6- and (MeS)(2)CSX+As2F11- (X = H, D) are reported.