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 compressibility is equal to the averageof the fluid adiabatic and isothermalcompressibilities weighted respectively by the heatcapacities of the fluid and mineral. On the otherhand, at the crossing of bubble point, in thelow-frequency regime a discontinuous variation influid compressibility exists. In these two relaxationprocesses, the difference in fluid bulk modulusbetween the unrelaxed and relaxed regimes, which isdirectly related to P-wave velocity dispersion, canbe as large as 0.5?1 GPa, depending on the fluid typeand gas saturation.