Two phase transitions were found in solid [Al(OS(CH3)(2))(6)](NO3)(3) at T-C1 = 343.1 K by differential scanning calorimetry and at T-C2 = 34.3 K by adiabatic calorimetry. The transition at T-C1 is accompanied by large thermal hysteresis (ca. 30 K) and a large entropy change (76J K-1 mol(-1)), suggesting an order-disorder-type transition, whereas that at T-C2 has a considerably smaller entropy change (3.4J K-1 mol(-1)). The phase behavior of the deuterated compound is essentially the same. IR and Raman spectra reveal a reduction in symmetry below T-C1 and a change in the reorientational dynamics of CH3 groups and NO3- anions, indicating that the highest temperature phase is orientationally disordered. The transition at T-C2 can be explained by a librational soft-mode theory and all complex cations change their orientation in a cooperative manner. (C) 2012 Elsevier B.V. All rights reserved.