Using first-principles calculations, we investigate the stability of composite systems consisting of BC3 nanotubes with a linear chain of bromine atoms inside. For the BC3 (n,n) nanotubes considered here (n = 2, 3, 4), we find that the binding energy per bromine atom exceeds its corresponding value in solid, liquid-phase, or gas-phase bromine. Charge transfer from the BC3 nanotube to the bromine chain results in hole pockets in the valence sigma bands of the BC3 wall.