Ultrasonic absorption measurements in the frequency range from 0.8 to 220 MHz were carried out in aqueous solutions of pentylammonium chloride (PEACL) and hexylammonium chloride (HEACL) with beta-cyclodextrin (beta-CD) at pH approximate to 7.2 and 25 degreesC. A single relaxational absorption was attributed to a perturbation of a chemical relaxation associated with the formation of a complex between beta-CD and the alkylammonium chlorides. The rate and equilibrium constants for the complexation reaction were determined from the concentration dependence of the relaxation frequency. Increasing the chain length of the alkylammonium ion led to an increase in the stability of the complex and slowed the exit rate of the ion from the beta-CD cavity. The standard volume change of the reaction was obtained from a maximum absorption per wavelength and was attributed to water molecules being expelled from the cavity with concomitant alkylammonium ion insertion.