Ultrasonic absorption coefficients in an aqueous solution of propylammonium chloride (PACL) and butylammonium chloride (BACL) (pH approximate to 7.2) were measured in the presence and absence of beta-cyclodextrin (beta-CD) at 25 degreesC in the frequency range 0.8-220 MHz. There was no relaxation in the solution of individual alkylammonium chloride or beta-CD. Also, in the solution with PACL and beta-CD, the relaxation was not observed. However, a single relaxational absorption was found in the solution of BACL when beta-CD coexisted. The cause of the relaxation was attributed to a perturbation of a chemical equilibrium associated with an interaction between beta-CD and the butylammonium ion from the concentration dependence of the relaxation frequency and the maximum absorption per wavelength. The rate and thermodynamic constants were determined for the association and dissociation reactions of the complex in the system of BACL with beta-CD. The results obtained were compared with those for other similar systems, e.g., propionate ion or propionic acid and alcohols with beta-CD. It was ascertained that the stability of the complex formed between BACL and beta-CD is relatively lower than those between other guests and beta-CD.