Abstract The dimethyl thallium(III) halides were investigated at low temperature and at high pressure by IR and Raman spectroscopy. These compounds have a tetragonal structure under ambient conditions, in which the methyl groups are disordered. Ordering transitions were observed at low temperature in all three halides. The low-temperature unit cells are monoclinic or triclinic with C2h or Ci symmetry and Z = 1 and Z = 2 per primitive unit cell for the chloride and for the bromide and iodide, respectively. At high pressure, the iodide and bromide underwent phase transitions at just below 10 kbar to phases similar to those observed at low-temperature. A second transition was observed in both the iodide and bromide at 27 and 45 kbar, respectively, which involved a change in conformation of the (CH3)2Tl+ ion from linear to bent and a distortion of the (TlX)nlayers. Significant spectral changes were also observed for the iodide and bromide at close to 65 kbar and 70 kbar, respectively, indicating the presence of another transition involving a change in methyl group orientation. The chloride underwent a transition at about 15 kbar at which the methyl groups become ordered. This phase appears to be related to that observed at low-temperature for the bromide and iodide. Further changes were observed at just below 25 kbar indicating that the (CH3)2Tl+ ions become bent. There is evidence for another transition above 35 kbar from changes in slope on plots of ν versus p.