Gas-phase electron diffraction patterns of 1,5-dicarba-closo-pentaborane and 1,6-dicarba-closo-hexaborane were recorded at room temperature. Least-squares analyses of the reduced intensity data confirmed theD,h and D4h symmetry of the respective carboranes in the gas phase. The carbon atoms are located at apical positions in the polyhedra and may be considered as bonded to a planar girdle of three and four boron atoms, respectively. The bond lengths for the 1,5-dicarbacloso-pentaborane (5) withD,h symmetry are B-C = 1.556 A, B-B = 1.853 A, C-H= 1.071 A, and B-H= 1.183 A. The bond angles are BCC = 73.05", CBB = 53.48", and CBC = 93.16". The distance from the apical carbon atom to the belt is 1.131 A. The bond lengths for the 1,6-dicarba-closo-hexaborane(6) withD,h symmetry are B-C = 1.633 A, B-B = 1.720A. C-H= 1 . 1 0 3 A , a n d B -H = 1.244 A. ThebondanglesareB,CB3=63.56",CBB=58.22",CBC= 83.73",andB2CB4=96.26". The distance from the apical carbon atom to the belt of four boron atoms is 1.090 A. Comparisons were made with related compounds whose structures were determined from microwave spectra or X-ray diffraction data.