We have studied the intriguing gas-phase dimerization of the B₁₂I_n⁻ (n = 9, 8) anions to B₂₄I_2n²⁻ dianions by means of density functional theory calculations. The dimerization of B₁₂I₉⁻ to B₂₄I₁₈²⁻ has been detected experimentally in a previous study (Phys. Chem. Chem. Phys., 2011, 13, 5712) utilizing electrospray ionization ion trap mass spectrometry (ESI-IT-MS), whereas the formation of B₂₄I₁₆²⁻ from B₁₂I₈⁻ is modeled here prior to experiment. Calculations are carried out to determine the molecular and electronic structures, the bonding situation and the stability of the dimers relative to the respective monomers. The dimerization process is found to be thermodynamically favorable, and the stability of the lowest-energy structures is substantiated by molecular dynamics simulations. The calculations imply that the experimentally observed B₂₄I₁₈²⁻ and the hypothetical B₂₄I₁₆²⁻ species are formed through dimerization of the respective B₁₂I_n⁻ (n = 9, 8) monomers, rather than through loss of two I radicals from B₂₄I_2n+2²⁻ dimers. Electronic properties such as natural charges, vertical detachment energies (VDEs), frontier molecular orbitals (FMOs), and HOMO–LUMO gaps are computed and analyzed in detail for all monomers and dimers. The analysis shows that the most stable B₂₄I_2n²⁻ dimers are formed through two 2c-2e B–B and two 3c-2e B–I–B bridges between the parent B₁₂I_n⁻ (n = 9, 8) monomers. These new bridging bonds engage the deiodinated (bare) faces of the two B₁₂ icosahedra, as well as one (per monomer) of the nearest boron neighbors and its iodine substituent.