Cp2MoS4, 1, in CH2Cl2 undergoes extensive oxidation by m-CPBA at -78 degrees C to give a 1,1,4,4-tetroxide, 2 (59%). Tetraoxide 2 rearranges at room temperature to give an unusual sulfur-atom protonated molybdocene disulfide dimer [(Cp2MOS2H)(2)](HSO4)(2), 3. Crystal data for 3 are P (1) over bar, a = 6.751(2) angstrom, b = 9.865(4) angstrom, c = 11.549(5) angstrom, alpha = 109.95(3)degrees, beta = 96.11(3)degrees, gamma = 108.22(3)degrees, V = 666.9(4) angstrom(3), and Z = 1. The dimer was also obtained directly when the oxidation was conducted at 0 degrees C in DMF Under basic conditions, the dimer is cleaved to give molybdocene disulfide Cp2MoS2, while treatment of the latter with acid converts it to the dimer 3.