Although 1,2-dehydro-o-carborane reacts with 3-hexyne to give the product of an ene reaction, it adds in both ene and 2 + 2 fashions to 1-phenyl-1-propyne to give 9 and 10, respectively. Single-crystal X-ray diffraction of 10 yields a structure for this three-dimensional analogue of benzocyclobutadiene. The stability of 10 shows that there is no strong orbital connection between the cage and the exo-cage pi system. Compound 10 reacts photochemically with 1,3-butadiene to give the 2 + 2 adduct 12.