Two phase transitions were found in solid [Al(OS(CH 3 ) 2 ) 6 ](NO 3 ) 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 (76 J K − 1 mol − 1 ), suggesting an order–disorder- type transition, whereas that at T C2 has a considerably smaller entropy change (3.4 J 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 CH 3 groups and NO 3 − 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.