The reaction of nido-6-SB9H10- with elemental sulfur results in sulfur insertion into the nido-SB9 cage framework to produce the new arachno-dithiaborane cluster anion, arachno-2,3-S2B9H10- (1(-)). A single crystal X-ray study has confirmed that, consistent with its formal arachno skeletal-electron count, the anion adopts a structure that may be derived from a 13-vertex closo-deltahedron by removal of two vertices. Crystal data for PSH(+)1(-) : a 9.294(1) Angstrom, b = 12.236(1) Angstrom, c 19.479(3) Angstrom, beta = 100.02(1)degrees, V = 2181.3(8) Angstrom(3), and Z = 4 in the monoclinic space group P2(1)/c. The two sulfur atoms adopt nonadjacent three-coordinate positions on the open six-membered face of the cage. In the solid state, a single bridge hydrogen was located at the B8-B11 edge; however, in solution, the NMR spectra indicate C-s cage symmetry suggesting that the bridging hydrogen is interconverting between positions at the B8-B11 and B9-B11 edges. The spectroscopic data indicate that, upon reaction of arachno-2,3-S2B9H10- (1(-)) with concentrated H2SO4 in CH2Cl2, protonation occurs at one of the cage sulfur sites to give arachno-2-H-2,3-S2B9H10 (1). Thermolysis of 1 in refluxing toluene results in dehydrogenation and formation of the previously known compound nido-7,9-S2B9H9. Deprotonation of 1 with Proton Sponge regenerates 1(-). 1(-) also reacts with CH3I or CH2I2 to give arachno-2-CH3-2,3-S2B9H10 (2) and arachno-2-CH2I-2,3-S2B9H10 (3), respectively. A single crystal X-ray structure determination of 2 confirmed that the methyl is bound to a sulfur atom and thus provides additional support for the structure proposed for 1. Crystal data for 2 : a = 7.733(1) Angstrom, b = 10.043(3) Angstrom, c = 13.630(2) Angstrom, beta = 101.84(1)degrees, V = 1036.0(6) Angstrom(3), and Z = 4 in the monoclinic space group P2(1)/c. In contrast to the structure observed for 1(-), in 2 the methylated sulfur adopts only a two-coordinate position bridging a boron-boron edge, while the other sulfur is in a three-coordinate position similar to those observed in 1(-). The similarity of the NMR spectra of 1, 2, and 3 suggest that 1 and 3 adopt cage structures similar to that confirmed for 2. Thus, either alkylation or protonation of 1(-) results in not only substitution at sulfur, but alters the nature of the sulfur-cage bonding interactions. The reaction of 2 with Proton Sponge results in the removal of the bridge hydrogen to yield the arachna-2-CH3-2,3-S9B9H9- (2(-)) anion.