Small pressure perturbations were introduced into the primary fuel pump bowl of the Molten-Salt Reactor Experiment (MSRE) operating at its nominal power of 8 MW(th). The experimental neutron flux-to-pressure frequency response was then obtained from a cross-power and auto-power spectral density analysis of the resulting signals from a neutron sensitive ionization chamber and a pressure transducer. By comparing the frequency dependence of the experimental frequency response determined for the reactor operating at power with the frequency response determined from analysis of mathematical models, the selection of the more appropriate boundary condition set from a choice of two possible boundary condition sets was possible. Then, the analytical frequency response was fitted by the least-squares method to the experimental frequency response to obtain the void fraction in the molten salt fuel. A void fraction of 0.61 ± 0.04% was determined from the frequency response; this value compares favorably with a value of 0.6 ± 0.1% determined by other techniques. Conclusions from this work are that the analytical model leads to acceptable results for the neutron flux-to-pressure frequency response and that properly designed dynamic tests involving small reactivity perturbations (introduced by means other than rod motion) can be used to extract specific nuclear parameters for a nuclear system operation at power.