Recent experiments by Hodak et al. [J. Phys. Chem. B 2000, 104, 5053] on core-shell nanoparticles have demonstrated that the frequency of the breathing mode is strongly dependent on the shell thickness. Here we compare these measurements with a theoretical model based on classical continuum mechanics, which in itself includes a model for the kinetics of shell formation. It is found that the experimental measurements can be modeled accurately using this theoretical approach. This finding will facilitate the control and characterization of the mechanical properties of core-shell nanoparticles.