A description is given of a technique for measuring small ultrasound velocity differences between two liquids, based on a new high resolution phase comparison method. The technique uses a two‐path acoustic system well suited for velocity dispersion measurements in the frequency range 3.3 to 330 MHz. The phase difference between two acoustic pulses traveling through the liquid is measured with a resolution of a few picoseconds. Velocity differences can be measured with an accuracy of ±1.5 cm/s. The unusually wide frequency capability and accuracy of the method are demonstrated with dispersion measurements on polystyrene samples dissolved in ethylbenzene.