Ultrasonic absorption coefficients in the frequency range of 0.8−95 MHz were measured in aqueous solution of leucine and glycyl-leucine in the presence and absence of β-cyclodextrin (β-CD) at 25 °C. A single relaxational absorption was found in both solutions although it was not observed in the absence of β-CD. The cause of the relaxation was attributed to a perturbation of a chemical equilibrium associated with an interaction between β-CD and the amino acid or the dipeptide. The rate and thermodynamic constants for the association and dissociation reaction of the complex in the system of glycyl-leucine and β-CD were determined from the concentration dependence of the relaxation frequency and the maximum absorption per wavelength. On the other hand, another analysis was applied for the system with leucine and β-CD to calculate the rate and thermodynamic constants due to the indistinguishable concentration dependence of the relaxation frequency. Therefore, first the equilibrium constant was estimated from ...