Differential scanning calorimetry (DSC) and low-temperature X-ray diffraction studies showed that 2-thio-(5,5-dimethyl-1,3,2-dioxaphosphorinanyl)2'-oxo-dineopentyl-thioph osphate (compound 1) undergoes reversible phase transition at 203 K related to the change of symmetry of the crystallographic unit. Solid state NMR spectroscopy was used to establish the dynamic processes of aliphatic groups and the phosphorus skeleton. C-13 and P-31 variable temperature NMR studies as well as T-1 and T-1 rho. measurements of relaxation times revealed the different mode of molecular motion for each neopentyl residue directly bonded to phosphorus. It is concluded that molecular dynamics of aliphatic groups causes different van der Waals interactions in the crystal lattice and is the driving force of phase transition for compound 1. Finally, we showed that very sharp phase transition temperature makes compound 1 an excellent candidate as a low-temperature NMR thermometer in the solid phase.