Ultrasonic absorption coefficients in aqueous solutions of 2-propanol in the concentration range from 1.00 to 3.99 mol dm(-3) have been measured over the frequency range from 3.0 to 220 MHz. In the solutions less than 3.00 mol dm(-3), no relaxational absorption has been observed. A single relaxational absorption has been clearly found at concentration of 3.00 mol dm(-3) or more. The results have been compared with those reported previously. The aqueous solutions of 2-propanol in the concentration range from 1.00 to 3.00 mol dm(-3) have been used as solvents in which a proton transfer reaction of propylamine has been examined in order to see the effect of additive on the reaction. The frequency dependence of the ultrasonic absorption coefficient in aqueous solutions of propylamine in the presence of 2-propanol at less than 3.00 mol dm(-3) has been well fitted to a usual Debye-type single relaxational equation. The absorption data in the solutions with 3.00 mol dm(-3) 2-propanol have been also characterized by the single relaxational equation when the relaxational absorption observed just in aqueous solution of 2-propanol is subtracted. The cause of the relaxation has been attributed to a perturbation on an equilibrium associated with the proton transfer reaction. The rate constants and the standard volume change of the reaction have been determined from the hydroxide ion concentration dependence of the relaxation frequency and from the maximum absorption per wavelength, respectively. It has been found that the forward rate constant decreases and reaches a constant value with an increase in the concentration of 2-propanol. The standard volume change of the reaction has slightly increased with 2-propanol concentration. Using theoretical equation for a diffusion controlled reaction, the diffusion coefficient of hydroxide ion has been estimated at various concentrations of 2-propanol. These results have been discussed in relation to the effect of 2-propanol on water structure, comparing with those in solvents with urea.