The molecular structures of two carbaboranes, closo-2,3-C2B9H11 and nido-2,9-C2B9H13, were determined experimentally for the first time using gas-phase electron diffraction (GED). For closo-2,3-C2B9H11, a model with C-2v symmetry was refined to give C-B bond distances ranging 158.3-167.0 pm and B-B distances ranging 177.4-200.0 pm. The structure of nido-2,9-C2B9H13 was refined using a model with C-s symmetry to give C-B bond lengths ranging 160.3-171.9 pm and B-B lengths ranging 173.0-196.1 pm. Ab initio computations (up to MP2/ 6-311+G*) were also carried out on these and the related nido-7,8-C2B9H13, which was not sufficiently stable to allow determination of its molecular structure by GED.