We have measured the sound velocity in liquid Se50Te50 at 19.5, 32.6, 45.6, and 58.2 MHz simultaneously by means of an ultrasonic pulse transmission/echo method. By using a phase-sensitive-detection technique the relative error was reduced to less than 0. 1%. The temperature dependence of sound velocity at 100 MPa exhibits a minimum at 600 degrees C and a maximum at 1035 degrees C, which are related to the large structural change accompanied by the semiconductor-metal (S-M) transition. In addition, sound dispersion has been observed at temperatures from 500 degrees C to 900 degrees C for the first time, where anomalous sound attenuation was previously reported. This result implies that a structural relaxation on nanosecond timescale takes place in the S-M transition region. (c) 2007 Published by Elsevier B.V.