The single-crystal X-ray diffraction structure of [(CH3)(4)N](2)PuCl6 is presented for the first time, resolving long-standing confusion and speculation regarding the structure of this compound in the literature. A temperature-dependent study of this compound shows that the structure of [(CH3)(4)N](2)PuCl6 undergoes no fewer than two phase transitions between 100 and 360 K. The phase of [(CH3)(4)N](2)PuCl6 at room temperature is Fd (3) over barc a = 26.012(3) A. At 360 K, the structure is in space group Fm (3) over barm, with a = 13.088(1) A. The plutonium octahedra and tetramethylammonium cations undergo a rotative displacement, and the degree of rotation varies with temperature, giving rise to the phase transition from Fm (3) over barm to Fd (3) over barc as the crystal is cooled. Synthesis and structural studies of the deuterated salt [(CD3)(4)N](2)PuCl6 suggest that there is an isotopic effect associated with this phase transition, as revealed by a changing transition temperature in the deuterated versus protonated compound, indicating that the donor-acceptor interactions between the tetramethylammonium cations and the hexachloroplutonate anions are driving the phase transformation.