Synthesis of the deltahedral cobaltadicarbaboranes CpCoC2Bn-3Hn-1 (n = 9, 10, 11, 12) typically leads initially to kinetically stable isomers with energies up to similar to 20 kcal/mol above the lowest energy isomers. Pyrolyses of these originally produced isomers typically results in isomerization to give more thermodynamically stable isomers. In this connection the relative stabilities of the CpCoC2Bn-3Hn-1 (n = 9, 10, 11, 12) isomers have been investigated using density functional theory. For CpCoC2Bn-3Hn-1 (n = 9, 10, 11) the isomers with both carbon atoms at degree 4 vertices are predicted to have the lowest energies. For CpCoC2B9H11 the icosahedron is by far the preferred polyhedron. Among the nine possible icosahedral CpCoC2B9H11 isomers, the unique isomer with the carbon atoms in antipodal (para) positions is the global minimum. However, the four CpCoC2B9H11, isomers with the two carbon atoms in mutual non-antipodal non-adjacent (meta) positions lie within similar to 5 kcal/mol of the global minimum. These theoretical results are in reasonable agreement with the extensive experimental work on pyrolysis of CpCoC2Bn-3Hn-1 (n = 9, 10, 11, 12) derivatives, mainly in the group of Hawthorne and co-workers during the 1970s.