The polymorphism of 2-bromo-2-methyl-propane ((CH3)3CBr) has been investigated by both thermal and X-ray powder diffraction experiments. The long time unknown structure of the low-temperature phase III has been determined by X-ray powder diffraction as orthorhombic Pmn21, with lattice parameters a = 19.1350(4) Å, b = 5.7378(1) Å, and c = 10.9626(2) Å at 180 K, Z = 8 and with an asymmetric unit containing one molecule in a general position and two additional molecules placed in a mirror (Z′ = 1 + 1/2 + 1/2). The phase transitions between the different phases have been characterized at normal pressure as well as at high pressures (up to 300 MPa). From the p−T slopes of the two-phase coexistence lines and density measurements as a function of pressure, the volume variations at the transition points have been obtained and compared with those obtained by means of X-ray powder diffraction at normal pressure. The packing as well as the anisotropy of the intermolecular interactions have been analyzed by means of the thermal-expansion tensor and compared with tert-butyl ((CH3)3CX, X = Cl, CN) related compounds.