The present proposal is part of a larger EPSRC project advancing the vibrational spectroscopy of silicate glasses. The project involves the techniques of IR and Raman spectroscopy, X-ray and neutron diffraction, and inelastic neutron scattering, in combination with molecular dynamics (MD) modelling of silicate glasses. Isotopic substitution of network modifier cations will provide a mass contrast by which the vibrational modes may be studied. The present proposal is to measure the inelastic neutron scattering of selected pairs of natural and isotopically enriched alkali silicate glasses; to observe the energy shift in the vibrational modes associated with the modifier cations; and to refine and validate MD models and the interatomic potentials used therein.