By means of a high pressure d&a. method, a new crystaliine phase of tetramethylsilane which forms below its melting point has been observed at pressures above 20 MPa. This is a verification of the prediction from earlier calorimetric data at ordinary pressures. l_ Introduction Recent calorimetric [ 1] and NMR [23 investigations on solid tetramethyisilane have straightened out the confusion [3--61 that existed in the literature as to the stability of different phases. Thus, the solid is trimorphous, and the Gibbs energies may be schematically depicted as in fig. I. The entropy of fusion of the ar phase (see table 1) Indicates that it is an orientationally disordered phase. From the phase diagram at ordinary pressures, the concealed solid-solid transition from the -y to the a phase was predicted to occur at 175.2 K, some 1.2 K higher than the triple point of the most stable r phase_ One would then presume that application of pressure would displace the 7 + a transition point downwards, allowing the a phase to exist as the stable phase immediately below the stable melting point. The present Ietter reports the results on the differential thermal analysis (d.t.a.) of solid Si(CH& made in examining such a possibility_