Cycloaddition reactions of 1,2-dehydro-o-carborane with thiophene, 2,5-dimethylthiophene, 2,5-dimethylfuran, [8](2,5)thiophenophane, and [8](2,5)furanophane are investigated. In all cases, except the reaction with the thiophenophane, both 4 + 2 and 2 + 2 cycloadducts are formed, with the 4 + 2 product dominating. The 4 + 2 adducts of the thiophenes are not stable but extrude sulfur under the reaction conditions to give "benzo-o-carboranes".