A series of nine- and ten-vertex nido and arachno boranes and heteroboranes have been studied by ab initio methods. At the highest level of optimization (HF/6-31G*), bond distances are in good agreement with X-ray data. The heat of formation of B10H14 is estimated to be -8.6 kcal/mol from a consideration of the reaction 10BH(3) - 8H(2) --> B10H14 With this value, heats of formation of other cages are calculated through near-isodesmic reactions.