Ultrasonic absorption coefficients were measured in aqueous solutions of imidazole and pyrazole by pulse and resonance methods in the frequency range from 3 to 220 MHz at 25 °C. Excess absorption was observed in both solutions. From the concentration dependence of the ultrasonic relaxation parameters associated with a single relaxational process in the solution of imidazole, the cause of the relaxation was proven to be due to a proton transfer reaction and the rate and equilibrium constants were determined for the process. The association rate constant for the ions was reasonable as a diffusion controlled reaction, although it is considerably greater than those in other amine solutions. A standard volume change of the reaction was also determined from the reactant concentration dependence of a maximum absorption per wavelength. The diffusion controlled rate constant for imidazole was considered in relation to the unique molecular structure of imidazole and the effect of the solute on water structure. In the solution of pyrazole, it was not possible to determine the relaxational parameters because the relaxation frequency was located quite below the measurement frequency range.