Ultrasonic absorptions in the 13-91 Mc/s region and ultrasonic velocities have been measured in molten pyridinium chloride containing various concentrations of anilinium chloride or cetyldimethylbenzylammonium chloride as solutes, at 155�. Adiabatic compressibilities were calculated for both systems. The anilinium chloride system behaved normally, but with the larger cation a sharp change in both absorption and velocity occurred at a concentration of 0.06m, in agreement with measurements of other physico-chemical properties. For more concentrated solutions, a pronounced ultrasonic relaxation was found, having a characteristic frequency of about 40 Mc/s; this may be interpreted in terms of a simple micelle growth mechanism.