Infrared and Raman spectra have been recorded with the aid of diamond anvil cells for all-trans-hexaphenylcyclohexasilane, [Si(H)Ph](6) (I), at various pressures up to similar to 40 kbar. The pressure dependences (d nu/dP) and relative pressure dependences (d In nu/dP) of the internal vibrations (nu(SiH),nu(SiSi),nu(SiC),nu(CH),etc.) have been determined. Two pressure-induced structural transitions were detected at similar to 5 kbar (phase I -> phase II) and similar to 18 kbar (phase I -> phase III), in which the conformation of the Sis skeleton and the orientation of the phenyl groups about the SiC bonds are changed. From the behavior of the internal vibrations of the Sis ring with increasing pressure, the (phase I -> phase II) structural transition most probably involves flattening of the cyclosilane ring. The internal vibrations of this ring and the phenyl groups have different pressure sensitivities. There is a pressure-scanned Fermi resonance in the IR spectra between the a(1) nu(CH) mode at 3054.0 cm(-1) and the A(1) binary combination (at similar to 3016.6 cm(-1)) of the two b(1) nu(CC) modes of the phenyl groups at 1584.0 and 143 3.0 cm(-1). The maximum Fermi resonance interaction occurs at similar to 12 kbar, in a good agreement with the similar to 11 kbar value predicted from the Lewis-Sherman equation. The positions of the a(1) CH mode and the A(1) combination at ambient pressure would be at 3045.0 and 3029.0 cm(-1) in the absence of Fermi resonance.