An interferometric technique has been developed for the purpose of measuring ultrasonic velocity and attenuation in rock melts. In essence, a long wave train is transmitted through two long buffer rods separated by a thin layer of melt. Multiple reflections in the melt layer interfere with each other and give rise to resonances for melt thicknesses equal to integral multiples of half the wavelength. The velocity and attenuation can be obtained by measuring the amplitude of the transmitted wave as a function of either the melt layer thickness or the frequency. The accuracy of the method was tested at room temperature by compressional wave measurements on water, mercury, and carbon disulfide. Velocities were found to be accurate to within 0.3%. Values of the specific dissipation factor Q-1 as low as 10-3 can be resolved for a liquid such as water with a low acoustical impedance, whereas Q-1 values as low as 10-4 can be measured for high-impedance liquids such as mercury. Q-1 values obtained on carbon disulfide agree with previous work to within better than 0.001. Experiments on a viscous organic polymer show that shear measurements can be made and that Q-1 values up to at least 0.2 can be measured by this technique. The method has been successfully used to study a basalt melt to 1500¿C. Sources of error and possible improvements are discussed, and the method is compared to previously used techniques. The method has the potential of measuring velocity and attenuation in partial melts and solids.