Abstract Ultrasonic absorption coefficients in aqueous solutions containing β-cyclodextrin and alcohols (ethanol and 1-propanol) have been measured in the frequency range from 3 to 220 MHz at 25 °C. Although a single relaxational absorption has been observed in solution with β-cyclodextrin and 1-propanol, it has not been found when the alcohol is ethanol. From the concentration dependence of the ultrasonic relaxation parameters, the cause of the relaxational absorption has been attributed to the perturbation of an equilibrium associated with the 1 : 1 complex formation (complexation) between β-cyclodextrin and 1-propanol. The rate and equilibrium constants as well as the standard volume change were determined for the complexation reaction of β-cyclodextrin and 1-propanol. The equilibrium constant determined in this study was found to be very consistent with the reported value for β-cyclodextrin and 1-propanol. The kinetic process for complexation between β-cyclodextrin and popular alcohols is discussed in relation to the sizes of the guest molecules.