A precise recording ultrasonic interferometer is presented which is particularly well suited for measuring dispersion in liquids. The device is an interferometer of the two‐crystal type in which the separation of the crystals is continuously varied. The recording process is equivalent to placing a mark on a rotating recording drum whenever the receiving crystal has moved a distance of one wavelength of sound in the liquid. Thus the wavelength of sound may be derived from the known mechanical constants of the system. The precision of the dispersion measurement is about 0.8 part in 10,000; the precision of the absolute determination of sound velocity is 0.2 m/sec. The instrument has been applied also to sound velocity measurements in solids under restricted conditions.Sound velocity in distilled water at 25°C as measured with this instrument is 1496.2±0.2 m/sec, a value somewhat lower than that obtained by earlier investigators.Dispersion in the frequency range 5–25 Mc is shown to be no greater than 0.8 part in 10,000, the limit of experimental error, for all the liquids studied: water, geraniol, xylene, isobutyl alcohol, and solutions of sucrose, NaCl,MgSO4, NaHCO3, CdCl2, CdSO4, Na3PO4, Pb(NO3)2, ZnSO4, andK3Fe(CN)6.