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 beta-cyclodextrin (beta-CD) at 25 degreesC. A single relaxational absorption was found in both solutions 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 beta-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 beta-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 beta-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 the concentration dependence of a maximum absorption per wavelength and then the rate constants were calculated. The results obtained were compared with that for the isoleucine and beta-CD system. In addition, the rate constant for the formation of the complex was discussed in relation to the diffusion controlled reaction.