Crystals of Ph4Si are tetragonal, space group P21c,a= 11·46(1), c= 7·09(3)Å, Z= 2, and are isostructural with Ph4Sn. The molecule possesses exact (S4) symmetry. The mean bond lengths (not corrected for libration) are: Si–C 1·383, C–H 1·00 Å. There is a significant deviation from local hexagonal symmetry of the phenyl group, involving a small displacement of the carbon atom attached to silicon. The structure was refined to R 0·073 for 413 independent reflections.