The normal mode frequencies and corresponding vibrational assignments of B 4 Cl 4 are examined theoretically using the Gaussian 98 set of quantum chemistry codes. All normal modes were successfully assigned to one of three types of motion predicted by a group theoretical analysis (B B stretch, B Cl stretch, B Cl bend) utilizing the T d symmetry of the molecule. The vibrational modes of the naturally isotopically substituted (1- 1 0 B, 2- 1 0 B, 3- 1 0 B and 4- 1 0 B) forms of B 4 Cl 4 were also calculated and compared against experimental data. A complex pattern of frequency shifts and splittings is revealed.