An electron diffraction study of 1,2-dicarba-closo-decaborane (10), 1,2C(2)B(8)H(10), constrained by ab initio calculations has been undertaken. A satisfactory refinement (R(G) = 0.052) was obtained with a bicapped "square" antiprismatic model assuming C-s symmetry. In this structure one carbon atom occupies one of the two capping positions, while the other is adjacent, in a basal position of a "square" pyramid. The experimental molecular dimensions (r(a)) are consistent with the r(e) structure as derived by the MP2(fc)/6-31G* optimization. The C-C edge [r(a) = 153.8(8) pm, r(e) = 153 pm] leads to distortion from the regular square pyramidal. shape. The compression of the carbon atoms toward the center of the cluster results in a substantial opening of the B(3)C(2)B(5) bond angle [both electron diffraction and MP2(fc)/6-31G* calculations give 95 degrees] from the parent 90 degrees. As a consequence, the C(2)B(3) distance is the longest CB nearest-neighbor separation so far observed in the gas phase, r(a) = 179.4(4) pm [cf. r(e) = 177.7 pm, MP2(fc)/6-31G*)]. The reliability of the experimental structure is assessed both by IGLO (individual gauge for localized orbitals) calculations of B-11 chemical shifts and by MP2(fc)/6-31G* single-point energy calculations.