The velocity of ultrasonic waves has been measured in the vapours of methyl fluoride, chloride, bromide and iodide, methylene fluoride and chloride, fluoroform and chloroform, carbon tetrafluoride and carbon tetrachloride, at frequencies 200, 566, 1192 and 400 kc/s, pressures ranging from 0$\cdot $25 to 2 atm, and temperature 100 degrees C. Dispersion, or incipient dispersion, occurs in all cases. Each dispersion zone observed corresponds to a single relaxation time, involving disappearance of the whole of the molecular vibrational energy. On the assumption that vibrational energy is taken up via the mode of lowest frequency, a simple functional relation is found to exist between the frequency of this mode and the probability of vibrational energy being acquired in collision. The theoretical significance of this is discussed. The paper includes second virial coefficient data for all the vapours investigated.