Ultrasonic attenuation was measured in dimethyl sulfoxide mixtures over the frequency and temperature range 5–200 MHz and 25°C to − 80°C. At the higher temperatures the quantity α/f2was found to be independent of frequency and in excess of the value calculated from the classical Stokes equation for shear viscosity. The excess attenuation can be attributed to a volume viscosity, which is two‐thirds of the shear viscosity. At the lower temperatures α/f2was found to be highly frequency dependent and less than that predicted by the Stokes equation. Furthermore, the data indicate that both shear and volume viscosities are relaxing with a single relaxation time. The low value of the excess attenuation observed at high tempeatures suggests that there are no specific hydrogen bonded complexes being formed. The dramatic decrease in α/f2with increasing frequency at low temperatures seems to indicate the onset of a glassy state.