The results of high-resolution helium atom scattering experiments on the dynamics of thin epitaxially grown films of KBr on NaCl(001) are presented and interpreted with a simple force constant model. The measurements reported are for two, three, four, and seven monolayers. The model treats the KBr overlayer with force constants obtained by fitting the bulk dispersion curves, the NaCl surface as a rigid wall, and the coupling between the two by a second set of force constants. The latter are determined by the fit to these data and are softer than the KBr force constants.