Poisson’s ratio is an important parameter when interpreting measured geophysical and seismic data. For an isotropic medium, it directly relates to the ratio of P- and S-wave velocities. We have measured as a function of pressure and frequency in fluid-saturated sandstones. The method of measuring was first tested as a function of pressure and frequency using standard samples. The phase shift between radial and axial strains was also measured. For all standard samples, such as the linear viscoelastic Plexiglas, the data indicated that correlated with and related to a dissipation on . Then, and were measured as a function of pressure and frequency for two dry and fluid-saturated Fontainebleau sandstone samples. Under dry conditions, no frequency dependence and very small pressure dependence were observed. Unusual behaviors were observed under fluid-saturated conditions. In particular, of one sample indicated a frequency-dependent bell-shaped dispersion under water and glycerin saturation that correlated with peaks in . Plotting the measurements as a function of apparent frequency (i.e., normalizing by the fluid viscosity) indicated a good fit between the water- and glycerin-saturated measurements. The bell-shaped dispersion in that was observed for one particular sandstone held for all effective pressures. These variations fully correlated with the peaks of observed. Our results can be interpreted using fluid flow and effective medium theories in the case of a porous microcracked rock. Drained/undrained and relaxed/unrelaxed transitions have frequency and magnitude of variations that are consistent with the measurements. The rock sample microcrack density strongly affects this frequency dependence. The inferred ratio at low effective pressures also indicates a large frequency-dependent bell-shaped dispersion. The parameter is a clear indicator of the frequency-dependent dissipation of and relates to the attenuation of P- and S-waves.