Experimental results are presented on the dispersive nature of elastic waves in porous rocks. These results show the combined effects of frequency, pressure, and fluid viscosity on the bulk modulus in fluid-saturated sandstone and basalt. At low frequency (0.1 Hz), samples behave as though fully drained: The bulk modulus remains unchanged under argon-, glycerin-, and water-saturated conditions. However, the high-frequency (or unrelaxed) bulk modulus, deduced from ultrasonic velocities, is clearly modified by the nature of the fluid. In addition, the bulk dispersion between low and high frequencies is significant at low confining pressure when cracks are open and decreases as cracks are closed. Although cracks represent a small fraction of total porosity, they produce a dispersion caused by squirt-flow processes. A simple quantitative model can be used to predict the frequency effect and explain the experimental results well.