Ultrasonic absorption coefficients in the frequency range 0.8-95 MHz were measured in aqueous solution of undissociated and dissociated propionic acid in the presence of beta-cyclodextrin (beta-CD) at 25 degreesC. A single relaxational absorption was found, although it was not observed in the absence of beta-CD. The cause of the relaxation was attributed to a perturbation of a chemical equilibrium associated with an interaction between propionic acid and beta-CD. The rate and thermodynamic constants for the association and dissociation reaction for the complex were determined from the concentration dependence of the relaxation frequency and the maximum absorption per wavelength. It was found that the complex formed by beta-CD and propionic acid is more stable when the acid is in undissociated form. Further, the ultrasonic relaxation associated with a proton-transfer reaction was also observed when no additive exists in the solution. The rate and thermodynamic parameters were also determined precisely.