Boron (III) halides (BX3, where X = F, Cl, Br, I) at ambient pressure conditions exist as strictly monomeric, trigonal-planar molecules. Using correlated ab initio calculations, the three heavier halides (X = Cl, Br, I) are shown to possess B2X4(mu-X)(2) local minima, isostructural with the diborane molecule. The calculated dissociation barrier of the B2I4(mu-I)(2) species [approximate to 14 kJ/mol with CCSD(T)/cc-pVTZ] may be high enough to allow cryogenic isolation. The remaining dimer structures are more labile, with dissociation barriers of less than 6 kJ/mol. All three dimer species may be stabilized by application of external pressure. Periodic density functional theory calculations predict a new dimer-based P (1) over bar solid, which becomes more stable than the P6(3)/m monomer-derived solids at 5 (X = I) to 15 (X = Cl) GPa. Metadynamics simulations suggest that B2X4(mu-X)(2)-based solids are the kinetically preferred product of pressurization of the P6(3)/m solid.