Two fluid-related sources of seismic attenuation andvelocity dispersion are examined: thermal relaxation, which originates from the contrasted temperaturevariations in the rock-forming minerals and in thesaturating fluid at the passage of the pressure wave;and the liquid-vapor phase transition in partiallysaturated rocks. An analysis of the relaxation timesshows that these processes are relevant in theseismic frequency band and drive the effective fluidcompressibility towards values higher than theunrelaxed values. Under full thermal relaxation between fluid and mineral, the effective...