Abstract: An experimental method to determine Poisson's ratio in a small beam subject to seismic excitation is presented. Poisson's ratio is computed by measuring longitudinal and transverse strains by means of electric strain gauges. A first set of tests is carried out with different materials, and it is observed that the measured Poisson's ratio decreases with frequency. However, to determine whether the observed decrease is true or it is due to an error caused by the plate effect of the beam, a second set of tests at different temperatures is carried out. Then, by applying the reduced variables method, a unique plot for Poisson's ratio on a much broader frequency range is obtained, which allows to state that the decrease of Poisson's ratio is true. An iterative procedure is described, which has been developed to gather the curves at different temperatures in a master curve.