A system of P-cyclodextrin and L-isoleucine in aqueous solution was studied by the ultrasonic relaxational method to obtain kinetic parameters for the complexation reaction at 25 degreesC. Ultrasonic absorption coefficients were measured as a function of frequency in the range 0.8-95 MHz and solute concentration at three different pH's. The frequency dependence of the absorption observed as a function of frequency was analyzed by a Debye-type equation with a single relaxation time at neutral pH. The cause of the relaxation was attributed to a perturbation of a chemical equilibrium due to formation of inclusion complex consisting of P-cyclodextrin and L-isoleucine. The forward and backward rate constants, the equilibrium constant, and the standard volume change of the reaction were determined from the ultrasonic parameters. Comparing these obtained results with those at low pH, it was found that the complexation reaction at low pH is the same as that at neutral pH range. However, at the high pH, another reaction is generated, in addition to the complexation reaction presumably due to a proton transfer reaction. Further, the results for the complexation reaction were compared with those reported previously for several nonelectrolytes, and the structure of amino acid was considered to influence the complexation reaction process considerably, especially the process for the departure of guest from the host cavity.