This paper presents the results of a project undertaken to study the axisymmetric natural vibrations and fluid-added mass of fluid-filled bellows expansion joints. The bellows were modeled using axisymmetric shell finite elements, while the fluid region was discretized using axisymmetric triangular elements. The in-vacuo bellows modes were used as boundary conditions on the potential flow model for the fluid and the added mass determined for each bellows mode. This added mass was then used to determine the in-fluid bellows natural frequencies. Experiments were conducted to verify the theoretical model and agreement was found to be very good. The results were also compared with the frequency predictions of previously developed relatively simple theoretical models.