The velocity of ultrasonic waves in sulfur hexafluoride (a symmetrical molecule with fifteen vibrational degrees of freedom) has been measured at frequencies of 251.22 kc and 1001.8 kc in the range from 0.38 to 15.89 Mc sec−1atmos−1at 36.10°C. The observed disperison in ultrasonic velocity is ascribed to the slow rate of energy exchange between translational and vibrational states. The dispersion region involves the disappearance of thewholeof molecular vibrational energy, and corresponds to asinglerelaxation time. The relaxation time as derived from dispersion measurements is found to be 5.89×10−7sec at one atmosphere corresponding to 4.68×103a collisions required for energy exchange. The relaxation time for a sulfur hexafluoride mixture with 0.066 mole fraction of helium is found to be 5.46×10−7sec.