The velocity of high‐frequency sound waves inn‐hexane vapor at 35.15°C has been measured at frequencies of 998.5 kc and 687.0 kc, and at pressures ranging from 1.10 to 12.28 cm Hg, using an ultrasonic interferometer. The dispersion which was detected in the velocity is attributed to the slow rate of energy exchange between the translational and vibrational states of the molecule. Although the upper limiting sound velocity could not be reached experimentally owing to weak response at low pressures and strong absorption at high frequencies, it was established that the inflection point in the velocityvsfrequency/pressure curve lay at 137 Mc sec−1atmos−1. This corresponds to a relaxation time of 6.46×10−9sec at one atmosphere if it is assumed that there is only a single dispersion region involving the relaxation of the vibrational modes as a unit. Finally, a value of 127 was found for the average number of collisions necessary to deactivate then‐hexane molecule. Measurements of the amplitude absorption coefficient were also carried out and the relaxation time derived from these measurements.