Bulk viscosity is rarely measured in the laboratory and is often poorly defined in the literature. We show that bulk viscosity can be a dominant factor in acoustic dispersion and attenuation of rocks containing a highly viscous fluid. A common method to measure attenuation and dispersion of seismic waves in the laboratory is the stress-strain method. We present bulk modulus attenuation and dispersion data collected on two heavy-oil-saturated rock samples by oscillating confining pressure method. Data was acquired at 0.001–1 Hz frequencies and compared to previous quasistatic axial stress-strain measurements. We demonstrate that bulk losses are significant even at these ultralow frequencies.