The gas-phase structure of 6,9-CSB8H12 has been determined by electron diffraction and ab initio calculations, and that of 6,9-CNB8H13 has also been calculated. The accuracy of each structure has been confirmed by 11B NMR calculations. The position of the sulfur atom is very close to that of the boron atom occupying the equivalent position in the parent molecule [B10H14](2-), reflecting the similarity of sizes of sulfur and boron atoms. The nitrogen and carbon atoms, on the other hand, lie much closer to the centers of the cages. The B8-X9-B10 angles increase from 98.7 degrees for X = S to 122.8 degrees for X = N. There are also large changes in relative lengths of bonds, with some bonds lengthening by up to 14.6 pm on introduction of a sulfur atom.