The compound 1-phenyl-1,2-dicarba-closo-dodecaborane(12), 1-C6H5-1,2-closo-C2B10H11 (1), has been synthesized and characterized by a complete assignment of its B-11 NMR spectrum via B-11(H-1)/B-11{H-1} (COSY), H-1{B-11(selective)} and H-1{B-11}/H-1{B-11} (COSY) spectroscopy. An electron- and X-ray diffraction investigation of 1, complemented by ab initio calculations, has been undertaken. The gas-phase electron-diffraction (GED) data can be fitted by several models describing conformations which differ in the position of the phenyl ring with respect to the carborane cage. Local symmetries of C-2v and D-6h for the 1,2-C2B10 and C-6 moieties, respectively, were adopted in the GED model in order to simplify the problem. In addition, constraints among the close-lying C-C and B-B bonds were employed. However, even though such simplifications led to satisfactory refinements (R(G) = 0.069-0.071), a unique, definitive solution could not be gained.The (C-C)(mean), (C-B)(mean), and (B-B)(mean) bond lengths, r(a), are ca. 1.44, 1.72, and 1.78 Angstrom, respectively. The C-6 hexagon, with r(a)(C-C) = ca. 1.394 Angstrom, either eclipses the C(1)-C(2) vector (overall C-s symmetry) or more or less eclipses the C(1)-B(4) cluster bond (overall C-1 symmetry). In contrast, in the solid at 199 K, the ring lies at a position intermediate between the two GED positions, as determined by X-ray crystallography [C8H16B10, monoclinic P2(1)/a: a = 12.047(3) Angstrom, b = 18.627(4) Angstrom, c = 12.332(5) Angstrom, beta = 110.09(4)degrees, Z = 8]. The C-B distances span the range 1.681(6)-1.743(5) Angstrom, and B-B lengths lie between 1.756(6) and 1.795(6) Angstrom. A similar conformation was found for the theoretical (RHF/6-31G* level) structure which was fully optimized in C-1 symmetry. The r(e) distances are consistent with the dimensions derived in the experimental studies. IGLO calculations of the B-11 chemical shifts, in addition to SCF single-point energies of the GED structures, further support these observations.