The electronic and geometric structures of homocubylidenehomocubane (HC), trans- (1-methyl-2-homocubyliden)-1-methylhomocubane (DMHC) and trans-(1-tert-butyl-2-homocubyliden)-1-tert-butylhomocubane (DBHC) molecules have been investigated by B3LYP/6-311G* method. These results agree with X-ray results and show that the double bond of HC molecule has a planar structure, while the double bonds in DMHC and DBHC molecule are almost planar. The complexes (1:1 71 complexes) of the molecules with Br-2 have been investigated by B3LYP/6-311+G* method and it is observed that the stable configurations have an axial structure. The electronic and the steric factors affecting the structure and the stability of the molecular complexes have been studied. It is observed that the DMHC...Br-2 complex is more stable than HC...Br-2 complex. The DBHC...Br-2 complex containing the bulky tort-butyl group has stability of sterically encumbered olefin-brom complex and it depends on the nature of 'cage' substituents on the double bond carbons. The bridged bromonium cation is the more stable cation among the cationic intermediates formed in the addition reaction of bromine to HC and the reaction occurs via this cation. The nucleophilic attack by bromide ion is not sterically prevented to HC-Br+ bridged bromonium cation and as a result, normal 9, 9'-dibromide product is formed. The nucleophilic attack by bromide ion is sterically prevented to AD-Br+ bridged bromonium cation.