The structure and vibrational spectra of the dimethyl sulfoxide solvated thallium(III) ion have been studied in a dimethyl sulfoxide solution and in the solid state. X-ray crystallography shows a trigonal unit cell, space group R3̄ (No. 148), for the [T](dmso)₆](ClO₄₎₃ compound TI(dMSO)61(CI04)3 Compound with Z = 3, a = b = 11.9764(13) [11.8995(9)] Å, c = 20.802(2) [20.467(2)] Å, and V = 2584.0(5) [2509.9(4)] ų at 295 [150] K. The crystal structure comprises a highly symmetric hexakis(dimethyl sulfoxide)thallium(III) ion, with thallium in a 3̄ symmetry site and a T1-O bond distance of 2.224(3) Å at 295 K. The octahedral T1O₆ kernel is compressed along the threefold axis with an O-T1-O bond angle of 96.20(11)°. The T1-O-S bond angle of 120.7(2)° corresponds to a T1···S distance of 3.292(2) Å. One perchlorate ion centered on the 3̄ axis was described by a statistically disordered model. A low-temperature EXAFS study (10 K) resulted in the T1-O and T1...S distances of 2.221(4) and 3.282(6) Å, respectively, consistent with a T1-O-S bond angle of 120(1)°. The low Debye-Waller factors confirm a regular coordination without the disorder of the dimethyl sulfoxide ligands, which would have resulted from the alternative choice of space group R3 for the crystal structure. Raman and infrared spectra have been recorded and assigned, with the bands at 435 and 447 cm^-1 corresponding to the vibrational frequency of the symmetric and asymmetric T1-O stretching modes, respectively. EXAFS data of a 0.5 mol dm^-3 thallium(III) trifluoromethanesulfonate in a dimethyl sulfoxide solution were consistent with that of a hexasolvated ion with mean T1-O and T1···S distances of 2.22(1) and 3.33(2) Å, respectively, which correspond to a mean T1-O-S bond angle of 124(2)°. The anomalously large disorder parameter for the T1-O distances is consistent with a weak pseudo-Jahn -Teller effect. The ²⁰⁵T1, ¹³C, and ¹H NMR spectra of the complex in solution show single signals at 1886, 39.5, and 2.3 ppm, respectively.