Ultrasonic absorption coefficients in aqueous solutions of urea at 1.50, 3.00, 4.00 and 6.05 M urea have been measured in the frequency range from 1.3 to 220 MHz at 25 degrees C. No excess absorption has been observed in these solutions, and the absorption decreased with increase of the concentration. The absorption measurements have been carried out in aqueous solutions of propylamine in the concentration range from 0.01 to 0.6 M, in which urea coexists. The excess absorption has been observed, and the frequency dependence of the ultrasonic absorption coefficient has been well fitted to a usual Debye-type single-relaxational equation. The cause of the relaxation has been attributed to a perturbation of an equilibrium associated with a proton transfer reaction. The rate constants have been determined from the hydroxide ion concentration dependence of the relaxation frequency, and the standard volume change of the reaction has been calculated from the reactant concentration dependence of a maximum absorption per wavelength. It has been found that the forward rate constant and the standard volume change of the reaction decrease with increase in the concentration of urea. On the other hand the backward rate constant has not been so affected by the addition of urea. It has been found that a linear relationship exists between the diffusion-controlled rate constant and the reciprocal viscosity coefficient in the solutions with urea. Using a theoretical equation for the diffusion-controlled reaction of ions, the diffusion coefficient of the hydroxide ion has been determined at various concentrations of urea. These results have been discussed in relation to the effect of urea on water structure.