The velocity of ultrasonic waves has been measured in a series of organic vapours at frequencies 566, 1192 and 4000 kc./sec., temperatures 20 and 100 degrees C, and pressures ranging from 0$\cdot $25 to 4 atm. Ultrasonic dispersion was found with benzene, cyclopropane and ethane. No dispersion was found with propane, n-hexane, cyclohexane, cyclohexene, ethyl chloride, chloroform, acetonitrile, acetaldehyde, acetone, diethyl ether and methyl alcohol. The mechanism and rate of the activation of vibrational energy by intermolecular collisions are discussed in relation to molecular structure. It is concluded that, for polyatomic molecules in general, there is little hindrance to the interconversion of translational and vibrational energy, especially at high temperatures. Slow interconversion, leading to ultrasonic dispersion, only occurs with small or rigid molecules, where internal rotations or vibrations of low frequency are absent. Interchange of vibrational energy between the different modes within the molecule is usually very rapid.