A technique for the direct measurement of the difference between the velocity of sound in two liquids is described. By choosing for a reference a dispersionless liquid, such as water, the technique becomes a sensitive method for measurement of the change of velocity with frequency in the test liquid. In practice the method is sufficiently sensitive so that changes in the velocity smaller than one part in 104 can be detected. Illustrative measurements are presented for an NaCl solution, sperm oil, and evaporated milk.