Three solid–solid phase transitions have been detected for [Sr(OS(CH3)2)4(NO3)2] at T = 229.3 K, T = 220.3 K, and T = 171.6 K (on heating) and at T = 226.6 K, T = 219.1 K, and T = 170.9 K (on cooling). The compound melts at Tm ≈ 347 K. Thermal behavior of the deuterated analog is essentially the same, but the phase transitions’ temperatures are shifted slightly towards higher temperature, by ca. 5, 5, and 15 K, respectively. Crystal structure in the lowest temperature of phase III at 93 K consists of completely ordered (CH3)2SO molecules and bidentate nitrate groups, which are coordinated to strontium cation. Infrared (IR) spectra reveal that the phase transition at T is associated with the slowing down of the reorientational motions of (CH3)2SO and NO3– groups and also with the lowering of crystal lattice symmetry of the compound. The extended solid-state vibrational analysis has been performed for both deuterated and protonated species by solid-state density functional theory computations and compared with IR and RS experimental spectra. Thermal decomposition occurs in two stages: in the first stage, in four steps, four (CH3)2SO molecules are liberated and Sr(NO3)2 is formed, and in the second stage, above ca. 800 K, SrO is formed.