Measurements of ultrasonic attenuation were made in a series of molten chlorides. The data indicate the existence of a structural volume viscosity whose activation energy differed from that for shear flow by from 1 to 6 kcal/mol. The ratio of volume to shear viscosity is the highest found so far (for an isothermal process) in any liquid, lying in the range 17 to 27. It appears that this ratio rises with the strength of intermolecular bonding in the liquid. The logarithm of this ratio of volume to shear viscosity was found to be linearly related to the entropy of fusion. Application of the Eyring‐Hirai “hole” theory of structural relaxation yielded reasonably good agreement with the measured values.