The aim of this work was to reveal the effect of butanoic acid in its dissociated form and undissociated form as a guest molecule on the kinetic parameters in an inclusion reaction with ${eta}$-cyclodextrin (${eta}$-CD). Ultrasonic absorption measurements in the frequency range from 0.2 to 45 MHz were carried out for ${eta}$-CD solutions with butanoic acid at $25^{circ}C$ by ultrasonic relaxation method. The rate and the equilibrium constants were obtained from the guest concentration dependence of the relaxation frequency, and the standard volume change of the complexation reaction were obtained from the maximum absorption per wavelength. A single relaxational absorption was observed, and the cause of the relaxation was attributed to a perturbation of the chemical equilibrium associated with a complexation reaction between ${eta}$-CD and butanoic acid. These results were compared with those in solutions containing both ${eta}$-CD and different guest molecules. It was found that the hydrophobicity of guest molecules played an important role in the formation of the inclusion complex and also that the charge on the carboxylic group had a considerable effect on the kinetic characteristics of the complexation reaction.