An improved synthetic route for the monocarbaborane arachno-4 -CB8H14, as well as the first definitive confirmation Of the structure of its conjugate anion, arachno-4-CB8H13- are reported. Thus, the reaction of the nido-7,9C(2)B(10)H(13)- anion and Me(2)S with the addition of concentrated hydrochloric acid has been found to give the 9-Me(2)S-mu-6,9-[(HO)BCH2]-arachao-6-CB9H11 zwitterion, in 58% yield. Further treatment of this intermediate species with a mixture of hexane and water results in the hydrolytic elimination of one CH and two BH vertices to give arachno-4-CB8H14 in a typical yield of 75% (45% based on the starting o-carborane). The structure of the arachno-4-CB8H13- conjugate anion was determined by both a single-crystal X-ray study and ab initio/IGLO/NMR calculations. The X-ray study confirms that in the solid state the anion has a 9-vertex arachno cage-geometry of C-1 symmetry with endo-hydrogens on C4 and B8 and adjacent bridge hydrogens at the B5-B6 and B6-B7 edges. This structure is also supported by the ab initio calculations which find that this configuration is the lowest in energy among those investigated. However, the IGLO calculated B-11 NMR chemical shifts for this structure do not match the reported room temperature solution B-11 NMR data for arachno-4-CB8H13-. Instead, the experimental spectra indicate a C, symmetry cage-structure containing three bridge hydrogens on the open face, suggesting the anion is fluxional in solution at room temperature. Good agreement between the experimental and calculated B-11 NMR chemical shifts was obtained by assuming a simple fluxional process involving rapid simultaneous migration of two hydrogens between bridge and endo positions and one hydrogen between two bridging positions. Averaging the IGLO calculated chemical shift values for the borons in the static structure that become equivalent in the fluxional process is found to give good agreement with the room temperature experimental B-11 NMR spectrum.