Infrared and Raman spectra of crystalline tetrakis(trimethylsilyl)silane(IV), Si[Si(CH 3 ) 3 ] 4 (TTMSS), have been recorded at various pressures ranging from ambient to ~55kbar with the aid of diamond-anvil cell (DAC). The vibrational data strongly suggest the occurrence of a phase transition at about 2-3kbar, which appears to be associated mainly with a change in the rearrangement of the trimethylsilyl groups. The tetrahedral Si Si 4 skeleton itself is not greatly affected by the application of high pressures. The ν(C H) and ν(Si C) modes of the attached trimethylsilyl groups are, however, particularly sensitive to pressure and exhibit significant changes in their pressure dependences (dν/dp) in both the IR and Raman spectra. The observed pressure dependences range from -0.61 to 2.28cm - 1 kbar - 1 . Differential scanning calorimetric measurements also indicate the occurrence of the phase transition fairly close to room temperature (-37.6 o C).