[Br 3 ][SbF 6 ] and [Br 3 ][IrF 6 ] were synthesized by interaction of BrF 3 with Sb 2 O 3 or iridium metal, respectively. The former compound crystallizes in the orthorhombic space group Pbcn (No. 60) with a=11.9269(7), b=11.5370(7), c=12.0640(6) Å, V=1660.01(16) Å 3 , Z=8 at 100 K. The latter compound crystallizes in the triclinic space group P (Formula presented.) (No. 2) with a=5.4686(5), b=7.6861(8), c=9.9830(9) Å, α=85.320(8), β=82.060(7), γ=78.466(7)°, V=406.56(7) Å 3 , Z=2 at 100 K. Both compounds contain the cation [Br 3 ] + , which has a bent structure and is coordinated by octahedron-like anions [MF 6 ] − (M=Sb, Ir). Experimentally obtained cell parameters, bond lengths, and angles are confirmed by solid-state DFT calculations, which differ from the experimental values by less than 2 %. Relativistic effects on the structure of the tribromonium(1+) cation are studied computationally and found to be small. For the heaviest analogues containing At and Ts, however, pronounced relativistic effects are found, which lead to a linear structure of the polyhalogen cation.