A detailed study of the vibrational structure of the silasesquioxanes H8Si8O12 and H10S10O15 based on INS, IR, and Raman spectra and on a normal-coordinate analysis is reported. The inelastic neutron scattering (INS) spectrum of crystalline H8Si8O12 is in good agreement with the optical data and allowed the assignment of optically forbidden transitions. The previously published force field, determined from IR and Raman data of H8Si8O12 and D8Si8O12, provided a good fit to the INS frequencies and intensities except in the O-Si-H bending region, which is sensitive to intermolecular interactions. The INS spectrum, a comparison of the Raman spectra of dissolved and solid H8Si8O12, and published diffraction data were used to analyse the influence of crystal packing. A modified force field was developed which is adapted to crystalline H8Si8O12 and distinguishes between O-Si-H bending force constants for the axial and the equatorial hydrogen atoms. Excellent agreement with all spectra was thus obtained. This force field was also applied to the INS spectra of H10Si10O15 and D8Si8O12. Based on a normal-coordinate analysis of the INS data, the total mean-square displacements (msd) of the hydrogen atoms of H8Si8O12 were extracted as well as limits for the total msd of the silicon atoms. The msd’s and the neutron diffraction values are in good agreement, showing that no or very little static disorder is present in crystalline H8Si8O12. Based on the msd’s, the lowest internal torsional frequency was estimated to be 41 +/- 7 cm(-1).