Ultrasonic absorption measurements in the frequency range from 1 to 95 MHz were carried out in aqueous a-cyclodextrin solutions with l-propanol or ethanol at 25 degrees C. A single relaxational absorption was observed, and the cause of the relaxation was attributed to a perturbation of a chemical equilibrium associated with a complexation reaction between alpha-cyclodextrin (host) and alcohols (guest). Rate and equilibrium constants for the complexation reaction were determined from the concentration dependence of the relaxation frequency for the solutions. A standard volume change of the reaction was also obtained from a maximum absorption per wavelength. These results were compared with those for the complexation between beta-cyclodextrin and some alcohols and were discussed in relation to cyclodextrin and alcohol molecular structures. It was found (1) that the formation rate of the complex was slightly dependent on the cavity size of the cyclodextrins, but the rate of departure of alcohols from the cavity of the cyclodextrins was very dependent on the structure of the cyclodextrins and alcohols, and (2) that a part of the hydrophobic group in alcohols incorpoarated into the cavity of cyclodextrins even if water molecules also participated in the complexation reaction between cyclodextrins and alcohols.